Pad/doc/architecture.md

# Runtime Architecture

This document describes how Pad actually works: the concurrency model, goroutine
responsibilities, the frame handoff contract, ownership rules, and the internal
design of the editor and browser. It is written at the level of *invariants and
contracts*, not line-by-line code, so it stays true as implementation details
evolve. If a detail below conflicts with the code, the code wins — and this
document should be fixed.

## 1. Concurrency model: single owner, no locks on state

The **logic goroutine is the sole owner (reader and writer) of mutable
`State`**. There is no `sync.Mutex`/`sync.RWMutex` on application state. Every
other goroutine talks to the owner through channels.

```
┌────────────────────────────────────────────────────────────────────┐
│ Main goroutine (Gio event loop)                                    │
│  - w.Event() loop: Config / Frame / Destroy                        │
│  - On FrameEvent: lock(handoff) → read Frame snapshot →            │
│    Renderer.Draw → collect gestures + key events →                │
│    e.Frame → unlock(handoff) → send inputs/config/query/layout    │
│    to logic channels (sends happen OUTSIDE the lock)              │
└───────────────▲──────────────────────────────┬─────────────────────┘
                │ handoff lock (frame storage) │ inputChan, configChan,
                │ + w.Invalidate()             │ layoutChan, searchQueryChan
┌───────────────┴───────────┐                  ▼
│ frameReceiver goroutine   │        ┌──────────────────────────────────────┐
│  tight loop:              │        │ Logic goroutine (SOLE OWNER of State)│
│   f := <-frameChan        │        │  - select over all input channels    │
│   lock; *frame = f;       │        │  - mutates State, builds []Element   │
│   w.Invalidate(); unlock  │        │  - sends Frame on frameChan          │
└───────────────────────────┘        │  - dispatches tasks to worker pool   │
                                     └───────────────┬──────────────────────┘
                                                     │ task dispatch / results
                                     ┌───────────────▼──────────────────────┐
                                     │ Worker pool (8 goroutines)           │
                                     │  - priority: high > medium > low     │
                                     │  - file I/O, directory index, line   │
                                     │    index build, autosave writes      │
                                     │  - results posted on ResultChan      │
                                     └──────────────────────────────────────┘
```

Key invariants:

1. **No locks on `State`.** The only mutex is the *handoff* mutex in
   `cmd/pad/main.go`, which guards the one-frame storage slot between
   frameReceiver and the main goroutine. It is a message-passing handoff, not
   a state lock; it is never held while touching a channel.
2. **The main goroutine never reads logic `State` directly.** It reads only
   the latest `Frame` snapshot (see §4) and writes only via channels.
3. **Non-owner goroutines never mutate `State`.** They send a request
   (channel) or run a callback on the owner (`Inspect`, test-only).
4. **The autosave timer goroutine reads nothing.** `time.AfterFunc` only sends
   a token on `autosaveChan`; the owner does all state reads and dispatches
   the write.
5. **Gio-mutable widget state lives in the main-goroutine-owned `Renderer`**,
   never in logic-owned `State` (see §5).
6. **Logic work is synchronous and fast.** Processing input, mutating state,
   building the element tree, and sending a frame must stay well under one
   display refresh (~16 ms). File I/O and index building always go to the
   worker pool.

## 2. Goroutine responsibilities

### 2.1 Main goroutine (`cmd/pad/main.go`)

- Runs the Gio event loop.
- On `app.ConfigEvent`: sends `ConfigEvent{PixelWidth, PixelHeight}` to
  `logic.ConfigChan()`.
- On `app.FrameEvent`:
  1. Reads `newScale = gtx.Metric.PxPerDp`; if it differs from
     `frame.Scale`, sends `ScaleEvent` (after the draw).
  2. Under the handoff lock: reads the `Frame` snapshot, calls
     `renderer.Draw(gtx, frame.Elems, scale)`, then `renderer.CheckGestures`
     and the focused element's key/edit events, then `e.Frame(&ops)`.
     Key events are queried with a catch-all `key.Filter{Focus: id}` **plus
     one named filter per arrow key**. This is required on Android: the
     window layer wraps plain arrow-key *presses* in `input.SystemEvent`
     (it wants them for focus navigation), and system events match only
     filters that name the key explicitly. Matching a named filter both makes
     the press deliverable and suppresses the focus-move side effect (a
     matched event makes `WakeupTime` report handled, skipping the window's
     `moveFocus`). The **shift key is tracked here** (`shiftDown`): Gio's
     Android JNI bridge never reads `KeyEvent.getMetaState`, so
     `key.Event.Modifiers` is always 0 and shift+arrow is otherwise
     indistinguishable from a plain arrow. `NameShift` press/release do
     arrive as plain events; the tracked state is attached to the
     `ui.KeyEvent{Shift: ...}` forwarded to the logic (OR-ed with the
     Modifiers field so desktop behavior is unchanged). Shift state is
     reset when no element is focused.
  3. Outside the lock: sends `[]ui.InputEvent` (if any) to `InputChan`,
     the search text (if it differs from `frame.Query`) to `SearchQueryChan`,
     and `renderer.GlyphLayout()` to `LayoutChan`.
- Owns `frameReceiver` (started in `run`).

### 2.2 Frame receiver

Tight loop: read `logic.FrameChan()`, store into the shared `Frame` under the
handoff lock, call `w.Invalidate()`. The logic goroutine's send on `frameChan`
(bufsize 1) never blocks for more than one frame cycle.

### 2.3 Logic goroutine (`internal/editor/logic.go`)

Sole owner of `*State`. `Run()` selects on:

| Channel | From | Payload | Action |
|---|---|---|---|
| `configChan` | main | `ConfigUpdate` (pixels or scale) | store scale/pixels; recompute layout |
| `inputChan` | main | `[]ui.InputEvent` | run each `Handler(evt.Data)` on the owner |
| `searchQueryChan` | main | `string` | store `Browser.Query`, re-filter, new frame |
| `openFileChan` | main (tap) | `string` path | open file in editor (chunked buffer + async index) |
| `layoutChan` | main | `ui.GlyphLayout` | store on editor state; derive `LastLineY` for scroll clamping |
| `retryChan` | logic | `string` filename | autosave retry |
| `autosaveChan` | timer | `struct{}` token | reconstruct content, dispatch `WriteFile` task |
| `workerPool.ResultChan()` | pool | task `Result` | apply async results (stat, read, index, write ack) |
| `inspectChan` | tests | `inspectReq` | run `fn(*State)` **on the owner** and reply (test-only) |
| `resultChan` | legacy | `ResultEvent` | drained (legacy channel; the live result path is the pool's) |
| `done` | main | — | drain pool, stop timer, exit |

After every state change the logic goroutine rebuilds the element tree and
sends a `Frame` on `frameChan`.

### 2.4 Worker pool (`internal/io/pool`)

Fixed 8 workers, two priority lanes (high > low; dispatch routes
`HighPriority` tasks to the high lane, everything else to low). Tasks are small
structs with `Execute() Result`; the `FileSystem` interface (`pool.FileSystem`)
has a real implementation (`io/pool/real`, rooted at `/`) and a mock
(`io/pool/mock`) for tests.

Task types in active use: `StatFile`, `ReadFile`, `BuildLineIndex`,
`WriteFile` (editor); `BuildIndex`, `LoadPages` (browser). Dormant/dead task
types — `ReadChunk` (no-op for fully-loaded in-range files), `ReadDir`,
`StatDir`, `ReadCache`, `WriteCache`, `Invalidate`, `SaveState`, `SaveUndo` —
are candidates for removal in a cleanup round.

## 3. Channel topology summary

- **main → logic:** `InputChan`, `ConfigChan`, `LayoutChan`, `SearchQueryChan`,
  `OpenFileChan`.
- **timer → logic:** `autosaveChan` (token only).
- **logic → logic:** `retryChan` (self-piped).
- **logic → frameReceiver:** `FrameChan` (the only outbound state carrier).
- **pool → logic:** `WorkerPool.ResultChan()`.
- **tests → logic:** `Inspect` (owner-executed callback; the production
  equivalent is "send a request channel message").

## 4. The Frame handoff contract

`editor.Frame` is the **only** data that crosses from the logic side to the
main side:

```go
type Frame struct {
    Elems            []ui.Element // element tree for the next draw
    Scale            float32      // current px-per-Dp
    FocusedElementID string       // which registered element gets key/edit events
    Query            string       // search text logic is filtering with
}
```

- `Elems` is the computed tree; the renderer draws it (it is a snapshot, safe
  to read from main).
- `Scale` lets main detect a density change and pass scale to `Renderer.Draw`.
- `FocusedElementID` drives which registered `key.Filter`/`key.FocusFilter`
  main uses to harvest keyboard/IME events.
- `Query` lets main compare against the (main-owned) search `widget.Editor`
  text and forward changes — the browser search box is a Gio widget, so its
  text lives on the main side, not in `State`.

## 5. Ownership rules

| Owned by | What |
|---|---|
| **Logic goroutine** | `State` (browser + editor + chunked buffers + line indexes), worker pool, autosave timer handle |
| **Main goroutine** | `*app.Window`, `op.Ops`, text shaper, `ui.Renderer` (gesture state, IME dedup state, glyph layout cache), the search-bar `widget.Editor` |

Rules:

1. **Gio mutates widgets during draw, so anything Gio mutates must be
   main-owned.** The search bar is a `widget.Editor` registered with the
   renderer by ID (`"search_bar"`); its text reaches logic only through
   `SearchQueryChan`.
2. **The editor content is NOT a `widget.Editor`.** It is the custom
   `ui.TextField` element; all editor state (buffer, cursor, scroll) is
   logic-owned. This is why the editor does not hit `widget.Editor`'s
   O(n²)-ish cost on large files.
3. **Per-frame element values are rebuilt by logic** (`[]ui.Element` in the
   `Frame`); any cross-frame persistent draw-side state (IME dedup, gesture
   tracking) belongs in the persistent, main-owned `Renderer`.

## 6. Editor internals (`internal/editor`)

### 6.1 Chunked buffer

`ChunkedBuffer` (`chunked_buffer.go`) is the edit buffer for open files.

- **Full load on open** for in-range files: the whole file is read into an
  ordered slice of chunks (64 KB) plus **prefix-sum byte offsets** over actual
  chunk lengths. There is no lazy loading and no eviction (both existed as
  plans and were removed).
- **Byte-indexed** throughout: `CursorPosition`, chunk offsets, and glyph
  `ByteOffsets` are byte offsets. All edit primitives are **rune-granular**:
  `HandleBackspace`/`HandleDelete` compute the UTF-8 rune width at the cursor
  (a byte-granular delete corrupts multi-byte characters, e.g. a two-byte
  character straddling a chunk boundary); IME edits arrive as rune ranges;
  tap-to-position lands on rune starts.
- Edits splice the affected chunk(s) only; chunks are not rebalanced.
- `LineIndex` (per-line byte offsets, `int32`) is built asynchronously by
  `BuildLineIndexTask` and stored on `cb.LineIndex` — the single source of
  truth (the old parallel `EditorState.LineIndex` field was removed). A file
  ending in `'\n'` has a trailing empty line (one offset past the last `\n`,
  equal to the file length).
- **Incremental line-index maintenance.** Every buffer edit updates the index
  in place instead of rebuilding it: `UpdateLineIndexAfterInsert(pos, text)`
  (shifts starts above `pos` right, keeps a start at `pos`, adds one start per
  inserted `\n`) and `UpdateLineIndexAfterDelete(start, end)` (drops starts in
  `[start, end)`, shifts the rest left, re-inserts `start` iff it is a line
  start in the new content). A start exactly at `end` always drops: it was
  created by the `'\n'` at `end-1`, which the deletion removes (line merge).
  These must be called in the same order as the buffer splice, and an IME
  replace is `Delete` then `Insert` at the same position.
- **Size guard:** files larger than `MaxEditableFileSize` (**50 MB**,
  measured on-device) open into a `TooLarge` state: the editor shows a notice
  and edit handlers are no-ops; the browser still lists the file.

### 6.2 Virtualized viewport

Only the visible byte range is shaped and drawn each frame:

- `VisibleByteRange` maps scroll offset + viewport height → `[startLine,
  endLine]` via the `LineIndex`, then to a byte range. The range is always
  bounded by real lines of the document.
- **Never shape the whole file.** Lesson learned (Phase 3): the shaper's
  internal `document` retains the backing array of its largest layout forever
  (`reset()` keeps the cap), so one whole-file layout permanently inflated
  memory to ~1 GB for a 10 MB file. Shaping ~50 lines keeps it small and flat.
- The renderer reports `GlyphLayout` back via `LayoutChan`; logic derives
  `LastLineY` for scroll clamping and the max scroll offset.
- **`GlyphLayout` offsets are window-relative.** The shaper only sees the
  visible window, so `GlyphLayout.ByteOffsets` (and `VisualLineStarts`, `Y`)
  are relative to `IMEWindowStartByte`, not the file start. Any logic-goroutine
  code that maps a glyph offset to the absolute `CursorPosition` (tap-to-position,
  Home/End, vertical cursor move) must add the window base
  (`IMEWindowStartByte`) before storing the cursor and subtract it before
  searching the offsets. For a whole-file window the base is 0 (a no-op).
  Getting this wrong snaps the cursor to the window top on scrolled large files.

### 6.3 Selection and caret

- Key presses arrive as `ui.KeyEvent{Name, Shift}` (main.go), not bare
  `key.Name`, so handlers can distinguish shift+arrow from plain arrow.
  On Android the `Shift` bit comes from main.go's own shift tracking, not
  from Gio's Modifiers (see §2.1); arrow-key presses reach the handler only
  because main.go registers the explicit named key filters.
- **Shift+arrow extends a selection**, plain arrow moves the caret and clears
  it. `SelectionAnchor` is the fixed end, `CursorPosition` the active end; both
  are **absolute file byte offsets**. No selection ⇔ `SelectionAnchor == -1`.
  A zero-length shift selection keeps the anchor (`SelectionStart/End == -1`)
  so the next shift-move extends from the original spot.
- Insert / backspace / delete with a live selection delete the whole selection
  first (`deleteRange`), then insert; the selection is cleared afterwards.
- IME: with an active selection, `HandleReplaceRange` unions the IME-reported
  range with the selection before splicing, so replacement is deterministic
  whether the IME reports the caret (empty range) or the full range.
- Rendering: the `TextField` element carries **window-relative** selection
  start/end into the visible `Value` (−1 = none) for the highlight, drawn
  before the glyphs; the IME `SelectionCmd` push dedups on the
  (selectionStart, caret) pair.

### 6.3a Touch selection (v1)

- **Division of labor: the renderer reports finger positions, the logic owns
  all geometry.** Touch input is delivered to the logic goroutine as
  `ui.Point` (tap), `ui.DoubleTapPoint`, `ui.LongPressPoint`, and selection
  drag events (which handle + app-local Dp position). The logic converts them
  to text coordinates using the `EditorRegion` stored on `State` (set each
  layout frame) plus the scroll offset, hit-tests the floating-menu items
  itself, and owns the menu rect, highlight range, and handle positions.
  The renderer only draws what the `Frame` snapshot says (menu, handles,
  highlight) and registers the input regions.
- **Menu taps are logic-decided.** The menu panel is one `Tap` interaction
  over its whole rect; the editor's tap handler ignores any tap inside a
  visible menu (`pointInMenu` guard) so both handlers can coexist regardless
  of dispatch order. Item identity comes from the tap's X within the panel.
- **Long press needs frames to elapse.** Gio renders on demand; a stationary
  finger produces no pointer events and therefore no frames, so the 400 ms
  threshold could never be checked. The main loop polls
  `Renderer.PendingLongPress()` and keeps invalidating the window while a
  press is held still on the editor. A non-grabbing raw pointer probe
  (plain `event.Op` tag on the editor region) observes the press's motion and
  cancels the pending long press on movement; a scroll or handle drag grabs
  the pointer (`pointer.GrabCmd`), which cancels it via `pointer.Cancel`.
- **A pointer filter with zero `Kinds` matches nothing.**
  `pointer.Filter.Matches` tests `e.Kind & f.Kinds == e.Kind`, so a query
  without `Kinds` silently receives no pointer events — probe queries must
  name the kinds they want.
- **One event per Update is a trap on Android.** A tap's down+up routinely
  arrive in a *single* frame, and `gesture.Click`/`gesture.Drag` return at
  most one event per `Update` call. If the renderer processed only one event
  per gesture per frame, the release would sit in the queue until the next
  redraw — which on an idle window may never come — and the tap is swallowed
  (this is why menu taps initially required a second tap to "rescue" the
  first). The renderer therefore drains each click/drag gesture's queue to
  exhaustion every frame; `gesture.Scroll` already drains internally.
- **Per-frame gesture bookkeeping must survive the frame.** Click-registry
  state (press time/position, long-press-fired flag) is kept in structs held
  **by pointer** in a map; `range` over a value-type map yields copies and
  silently discards the mutations.
- **Clipboard crosses the goroutine boundary via channels; the ops run on
  the main/Gio frame path.** Logic→main: `clipboardSetChan` (string to write)
  and `pasteReqChan` (token). Main→logic: `pasteChan` (string). All three are
  buffered (16) so a harness with no main loop can never block the logic
  goroutine. Main executes `clipboard.WriteCmd` / `clipboard.ReadCmd` during
  a frame and forwards read results back on `pasteChan` from the
  `transfer.DataEvent`.
- **Android clipboard reads need an explicit invalidation.** Gio v0.10 on
  Android answers `ReadCmd` synchronously during the op flush by queueing a
  `transfer.DataEvent`; a queued DataEvent schedules **no** frame wakeup of
  its own. Main therefore invalidates the window after each read so a
  follow-up frame exists in which the DataEvent is consumed.
- **The menu closes on any item tap.** Copy keeps the selection (only the
  menu disappears); cut removes it (via `ClearSelection`, which also hides
  the menu and cancels drag bookkeeping); paste closes the menu immediately
  even though the actual insert happens on a later frame when the clipboard
  content arrives.

### 6.4 IME (Android soft keyboard)

- The editor exposes to the IME a **windowed snippet**: `IMEWindowText` is the
  visible viewport text, `IMEWindowStartByte` its absolute start. While the
  `TextField` is focused, `drawElement` emits `key.SnippetCmd` (the window)
  and `key.SelectionCmd` (caret as a rune index into the window).
- **Dedup in the Renderer** (main-owned state): the snippet/selection are
  re-emitted only when they actually change, and a fresh push is forced when
  the field (re)gains focus. This mirrors `widget.Editor`'s behavior and
  prevents per-frame re-push from resetting IME composition on rapid commits.
- Incoming `key.EditEvent{Range, Text}` has **window-relative rune
  indices**; `HandleReplaceRange` converts them to absolute byte offsets
  (`RuneIndexToByte`) and splices the chunked buffer. Insertion,
  `deleteSurroundingText` (backspace/autocorrect replacement), and composition
  all arrive through this one path.

### 6.5 Autosave

- Any edit calls `markDirty()`: a 1 s debounce timer; each keystroke restarts
  it. On expiry the timer goroutine sends a token on `autosaveChan`; the owner
  reconstructs the full content from the chunked buffer and dispatches a
  `WriteFile` task.
- Write failures are tracked per file (`writeFailed`, `retryAttempts`) and
  retried via `retryChan`. There is no save button; autosave is the only
  persistence.

### 6.6 Opening a file

- A browser tap sends the path on `OpenFileChan`. The logic goroutine creates
  the `ChunkedBuffer`, dispatches `StatFile` (size guard) and the read +
  `BuildLineIndex` tasks, switches `page` to the editor, and sets
  `justOpenedAt`.
- **Opening-tap swallow:** the tap that opens the file is also delivered as an
  editor tap in the same frame. A short time window (`justOpenedAt`) swallows
  it so the viewport does not jump to the tapped (often EOF) position.

## 7. Browser internals (`internal/browser`)

- `BrowserState` is **embedded by value** in `State` (single owner; no
  pointer indirection).
- `BrowserManager` drives async directory loading through the worker pool:
  `ReadDir` + `BuildIndex` on navigation, `LoadPages` for pagination.
- Features: 4 sort modes (name/date × asc/desc; default newest-first),
  incremental case-insensitive search over names, directory navigation with
  back, single-tap file open.
- The browser renders via `ListView`/`ListItem` elements; rows carry
  `Interaction` handlers that send channel requests (tap) or mutate state
  directly when the handler runs on the owner (scroll).

## 8. Render pipeline (`internal/ui`)

- **Logic** builds a `[]ui.Element` tree. Elements are value types; interactive
  ones carry `Interaction{Gesture, Handler}` entries. `Handler` is a static
  function that mutates `TheState` — handlers run on the logic goroutine only.
- **Main** draws via `Renderer.Draw(gtx, elems, scale)`:
  1. `drawElement` recursively walks the tree, applying transforms/clips.
  2. Interactive elements register hit regions in the *current clip context*
     (`gesture.Click.Add` / `pointer.InputOp`), so hit-testing always matches
     the drawn geometry.
  3. Text elements shape through the shared shaper — **only the visible
     window**, never the whole buffer.
  4. `e.Frame(&ops)` flushes ops.
- After draw, `CheckGestures` turns raw pointer/gesture state into
  `[]ui.InputEvent` (each carrying its own handler) for the logic channel.
- **Units:** `ui.Dp`/`ui.Px` convert via the current `PxPerDp` scale
  (`ToDp`/`ToPx`). The window is 390×844 dp; the real pixel size arrives via
  `ConfigEvent`.
- **Live element catalog:** `Container`, `Label`, `Icon`, `TextField` (editor
  window), `ListView`/`Line`/`ListItem` (browser rows), `GioEditor` (search
  bar), `Cursor`. Types `Button`, `Toast`, `Spacer`, `AlphaIndex`,
  `Selection`, `MergeHunk` exist in `element.go` but are not used by any page
  — future work, not current behavior.
- **Theme:** `ui.Theme{FontSize: 14}` + palette in `element.go`.

## 9. Testing hooks

- `Logic.Inspect(fn)` (test-only): runs `fn(*State)` **on the owner** and
  returns its result — the sanctioned way for tests to read state without
  breaking single ownership. It must never be called from production code.
- The e2e harness (`internal/test/e2e`) drives the **real** `Logic` + worker
  pool + mock filesystem through the real channels, and asserts via
  `Inspect`. It is logic-only: it does not exercise `Renderer.Draw`, so
  draw-path behavior (IME dedup, gesture routing) is validated on-device with
  adb (see `development_plan.md` §Phase 2/3 observation loop).
- CI gate: `go build ./... && go vet ./... && go test -race ./...`.

## 10. Known loose ends (code, not spec)

- Globals `TheState` / `TheLogic` / `ui.OpenFile` still exist (handlers are
  static funcs); replacing them with explicit state is a planned cleanup
  (`development_plan.md` Phase 4).
- Dead task types (`ReadChunk`, `SaveState`, `SaveUndo`, …) and unused element
  types are candidates for removal in the same round.
- The Android arrow-key/shift workaround in main.go (named `key.Filter`s +
  app-side shift tracking, §2.1) compensates for two Gio v0.10 behaviors: the
  JNI bridge dropping modifier state, and mobile arrow presses being wrapped
  in `input.SystemEvent` for focus navigation. If Gio changes either
  behavior (e.g. starts passing meta state), the shift tracking and the named
  filters must be re-examined — the workaround would become redundant or
  wrong. Re-verify on-device with `adb shell input keyevent 22` (cursor must
  move) and `input keycombination 59 22` (selection must extend).

## 11. Performance profiler (default-off, `internal/perf`)

Pad ships a built-in profiler that is **off by default** and costs nothing when
off. It is the in-app frame-timing tool (see `development_plan.md` §9: `gfxinfo`
cannot measure this app because it renders into a `SurfaceView`).

- **Enable** by creating the marker file
  `/storage/emulated/0/PadPerf/enable` before launch. `main.go` then creates a
  `perf.Profiler` that writes `logic_frames.csv` (one row per logic frame: seq,
  ms-since-start, frame delta, page, scroll Dp, max-scroll Dp, total lines,
  visible byte range) and logs a rolling ~1 s `PERF` summary. `DestroyEvent`
  stops it (final flush + `PERF stopped` summary with p50/p90/p99/max).
- **Hook**: `internal/editor.PerfRecord` is a package-level func, set by
  `main.go` only when enabled. `Logic.emitFrame` calls it on the owner
  goroutine with a `ProbeRecord` **before** sending the frame. When disabled it
  is `nil` and the per-frame cost is a single nil check.
- **Debug scroll jumps** (for testing, off by default): with the profiler on,
  `main.go` also polls `/storage/emulated/0/PadPerf/cmd` (a one-shot file
  consumed on read). `top`, `bottom`, `frac <0..1>`, and `dp <int>` jump the
  editor's `ScrollOffset` (clamped to `[0, MaxScroll]`) and emit a frame. This
  lets a test drive large-offset scrolls deterministically without pixel taps.
- The profiler is owned by the goroutine that creates it and is single-goroutine
  (no locks). It does **not** `Sync()` the CSV per flush (only per row batch) to
  avoid periodic fsync hitches in the logic path.
- **Measured** (emulator, 10 MB file, 2026-08): logic-frame cadence is flat
  across scroll offsets 0.02→1.0 (no large-offset degradation); the visible byte
  range stays ≤ ~4.3 KB (0.04% of the file); PSS plateaus ~250 MB (bounded
  high-water mark, no leak). See `development_plan.md` Phase 6.