Finish Phase 1 items 1, 2, 4, on top of the range-handling done in 9b78219:
- TextField.Draw now emits, when focused:
* key.InputHintOp{HintText} (item 4: enable text keyboard/autocorrect)
* key.SnippetCmd (the visible window as the snippet, Range {0,len})
(item 2: swipe/autocorrect source)
* key.SelectionCmd (caret, window-relative rune index)
(item 1: IME selection sync)
The snippet is the visible window (not the whole file), so the IME treats
the window as the document and reports EditEvent.Range window-relative.
- HandleReplaceRange now resolves the window-relative range against
IMEWindowText and offsets by IMEWindowStartByte to address the buffer
(string and chunked paths). Falls back to the whole buffer when layout
has not set the window (tests).
- EditorState gains IMEWindowStartByte / IMEWindowText, set during layout.
- Add runeCount helper (utf8 leading-byte scan) in the ui package.
- Fix a data race in three editor e2e/integration tests: they called
OpenFile from the test goroutine while the logic goroutine ran layout;
now wrapped in withState so the state write happens on the owner.
Tests: ime_range_test.go gains windowed-path coverage (string+chunked).
go build, go test, go vet, and go test -race are all green.
The IME commit path (swipe-to-type, autocorrect replacement) sends a
key.EditEvent with a Range that the editor ignored, causing the replaced
region to be duplicated. Add Logic.HandleReplaceRange which deletes the
rune range [start,end) and inserts text, converting rune indices to byte
offsets (string path: utf8.DecodeRuneInString; chunked path: leading-byte
scan). HandleKeyDown now routes key.EditEvent through it.
Also fixes two pre-existing ChunkedBuffer correctness bugs the tests
exposed:
- Delete mis-computed the per-chunk end using a shrinking 'remaining'
instead of the absolute end, corrupting multi-chunk deletes.
- FullContent derived its chunk bound solely from fileLen, truncating
in-memory chunks grown past the old bound by an insert.
Adds ime_range_test.go covering insert/replace/delete, unicode, chunked,
and swapped bounds. go test -race ./... green.
