gtauri: fix deep-scroll stall + instant viewer

Root causes found via per-event tracing (trace.log) + heartbeats:

1. Decode backlog: single worker at ~300ms/photo could not keep up with
   fast long scrolls — phone session ended with a 2400-deep queue
   draining at ~3 cells/s (13+ min to catch up). That was the
   'scroll far down and nothing loads no matter how long I wait' zone.
   - 4 parallel decode workers (~12 photos/s)
   - viewport-aware stale drops: prefetch jobs more than 60 rows behind
     the viewport are dropped, not decoded (fleeing leaves no backlog)
   - Q_CAP bounds the queue; JS 1.5s safety net re-requests
   - queue peak in repro test: 42 (was 2405); all 850 decodable
     emulator images now load (708 before)

2. Stale-anchor bug: sendAnchor was leading-edge throttled with no
   trailing call — if a scroll ended inside the 150ms window the Rust
   anchor stayed stale forever, and the worker then dropped fresh
   URGENT jobs (tap → full read) as 'off-screen'. Fix: trailing
   anchor send; and URGENT jobs are never stale-dropped (only
   prefetch work is droppable).

3. Viewer latency: full-res read ran on Chromium's main thread
   (wry shouldInterceptRequest is synchronous) — 30-50MB flash read
   froze the whole WebView. Now: thumb shown instantly (mem cache,
   ~1ms), full read queued urgent and done off-main; full-ready event
   swaps in the full-res (256MB full-res LRU makes re-opens instant).
   Measured click→full-res: 4ms on emulator.

4. Scan: skip OS junk (._* AppleDouble forks, dotfiles) — the phone's
   DCIM carried 72 of them, rendering as permanent gray cells.

5. Viewer for a corrupt file now shows 'image unavailable' instead of
   'loading…' forever; grid retry timers skip far-off-screen cells.

harnesses/gtauri/Cargo.lock

@@ -346,6 +346,12 @@ dependencies = [
  "cc",
 ]
 
+[[package]]
+name = "color_quant"
+version = "1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3d7b894f5411737b7867f4827955924d7c254fc9f4d91a6aad6b097804b1018b"
+
 [[package]]
 name = "combine"
 version = "4.6.9"
@@ -1095,6 +1101,16 @@ dependencies = [
  "r-efi 6.0.0",
 ]
 
+[[package]]
+name = "gif"
+version = "0.14.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ee8cfcc411d9adbbaba82fb72661cc1bcca13e8bba98b364e62b2dba8f960159"
+dependencies = [
+ "color_quant",
+ "weezl",
+]
+
 [[package]]
 name = "gio"
 version = "0.18.4"
@@ -1556,11 +1572,24 @@ checksum = "85ab80394333c02fe689eaf900ab500fbd0c2213da414687ebf995a65d5a6104"
 dependencies = [
  "bytemuck",
  "byteorder-lite",
+ "color_quant",
+ "gif",
+ "image-webp",
  "moxcms",
  "num-traits",
  "png",
 ]
 
+[[package]]
+name = "image-webp"
+version = "0.2.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "525e9ff3e1a4be2fbea1fdf0e98686a6d98b4d8f937e1bf7402245af1909e8c3"
+dependencies = [
+ "byteorder-lite",
+ "quick-error",
+]
+
 [[package]]
 name = "indexmap"
 version = "1.9.3"
@@ -2498,6 +2527,12 @@ version = "0.1.30"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d55d956fa96f5ec02be2e13af0e20391a5aa83d6a074e3ad368959d0fab299ea"
 
+[[package]]
+name = "quick-error"
+version = "2.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a993555f31e5a609f617c12db6250dedcac1b0a85076912c436e6fc9b2c8e6a3"
+
 [[package]]
 name = "quick-xml"
 version = "0.42.0"
@@ -4104,6 +4139,12 @@ dependencies = [
  "windows-core",
 ]
 
+[[package]]
+name = "weezl"
+version = "0.1.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a28ac98ddc8b9274cb41bb4d9d4d5c425b6020c50c46f25559911905610b4a88"
+
 [[package]]
 name = "winapi"
 version = "0.3.9"

harnesses/gtauri/Cargo.toml

@@ -24,4 +24,4 @@ rusqlite = { version = "0.41", features = ["bundled"] }
 turbojpeg = { version = "1.5.1", features = ["cmake"] }
 kamadak-exif = "0.6"
 sha1 = "0.11"
-image = { version = "0.25", default-features = false, features = ["png"] }
+image = { version = "0.25", default-features = false, features = ["png", "webp", "gif"] }