fix(editor): smooth writing during autosave and pan/zoom (1.3.5)

[markm39]

Fixes the 1.3.4 regression: strokes lagged ~1 s while writing (during autosave), and pan/zoom was very laggy.

Changes

• Engine 0.3.9 (fix(ink): responsive pen input during saves; no per-frame re-render on pan/zoom (0.3.9) mobile-ink#60): stroke snapshot + native captureDocument off the main thread; sticky overscanned render target so pan/zoom doesn't re-render ink each frame. Pin lands once 0.3.9 is published.
• Editor no longer re-renders ~60×/s during pan/zoom. useViewportTransformStore subscribed the whole note screen with useSyncExternalStore; it now only creates the store (consumers already subscribe directly).
• Autosave never overlaps saves (autosaveQueue): a debounced save that fires while another is in flight waits for it, so an older capture can't land after newer ink. A save superseded by new ink reports pending (typed engine error), real failures still report error.
• Version 1.3.5 (18).

Evidence (iPad Pro 13" M5 simulator, Release, 100-page dense fixture — not physical iPad)

	1.3.4	this PR
Main-thread stall per autosave, median / max	59 / 63 ms	6 / 9 ms
Scroll + zoom test app CPU	3.47 s	1.68 s
Background save CPU (35 s writing)	7.46 s	2.36 s

New testDenseWritingDuringAutosavePerformance writes strokes 400 ms apart so every stroke overlaps a save.

Tests

• npm test 116 pass (new autosaveQueue.test.mjs: no overlapping saves, superseded vs real failure), tsc clean.
• All 11 OpenNotesUITests pass on the Release simulator build: cross-page stroke undo/redo/save/relaunch/PDF export, text/asset history, image crop, dense audit, invalid payload recovery, performance cases.