feat(canvas): replace Canvas 2D with a native recording-backed architecture

[BibekPathak]

Deliver the complete Chromium-style Canvas 2D re-architecture for Moli,
replacing the scattered pixel-ownership model with a native, recording-backed
core in moli-canvas. This is the full M0–M6 migration: baseline, geometry
extraction, surface/backend, single-owner integration, ordered recording,
observation/invalidation, and removal of the old implementation.

Crate architecture

• moli-canvas becomes the browser-independent native core: drawing state,
  current-path geometry, ordered recording, persistent premultiplied-RGBA8
  surface, region readback, and encoding/export helpers. It depends only on
  AnyRender/Vello-CPU/peniko/kurbo/mol-image; it never touches V8, the DOM,
  layout, or page paint.
• moli-renderer-v8 remains the browser adapter (WebIDL, JS identity,
  invalidation, page-image publication, lifecycle). moli-paint responsibility
  for page painting is unchanged.

Milestones

• baseline cost model (per-draw full-plane copy is O(canvas area)) and the
  routing/ownership inventory used as the final-review checklist.
• native CanvasPath + per-context path state in moli-canvas; renderer
  adapter reduced.
• CanvasSurface (premultiplied RGBA8, lazy materialization, reusable
  Vello CPU backend, cached straight-alpha snapshot, region readback,
  reset/resize) with native surface/backend tests.
• a single weak-keyed native surface replaces the mutable V8
  Uint8ClampedArray backing store; GC/isolation reclamation, resize, and all
  115 canvas JS regressions pass.
• ordered DrawRecording captures every ordinary draw op (path fill/stroke,
  rect fill/stroke, clear, drawImage, text, putImageData) with frozen inputs,
  batching contiguous scene ops into one backend submission while preserving
  order through direct-step segments for clears/blits/writes. Integrated into
  the renderer; dead rasterize/composite/color helpers removed.
• consistent flush-before-read on all pixel observations, snapshot-cache
  invalidation on every write path, and VisualResourceGeneration bumped on
  every draw record (not just at flush) so screenshots/screencast see current
  content and geometry-only ops neither flush nor invalidate.
• remove the full-surface unpremultiply/premultiply round-trip from all
  four direct ops by compositing in premultiplied space (O(operation area) not
  O(canvas area)); delete CanvasSurface::with_straight_pixels_mut; verify
  same-size reset; declare STUBs out of scope.

Correctness hardening

• putImageData now does a true raw overwrite (premultiplied straight-alpha
  replacement) instead of an accidental source-over composite, matching the
  spec even for translucent ImageData over existing content.
• Add a pending-recording resource budget: after a draw, flush early when
  recorded bytes (including pinned source images) exceed the limit, mirroring
  Chromium's FlushIfRecordingLimitExceeded.

Checks

• cargo fmt --all, clippy (moli-canvas, moli-renderer-v8) with -D warnings, and
  the moli-canvas native suite (45 tests) pass.

[BibekPathak]

Hey @ldm0 , look at this!!

[ldm0]

Apply globalAlpha only to the alpha channel of this straight-RGBA color. Scaling RGB here causes Vello to premultiply already-darkened channels. A red path with globalAlpha = 0.5 reads back as [128,0,0,128], whereas its straight RGB should remain red, approximately [255,0,0,128]. This affects fill, stroke, and strokeRect. Please add a regression that checks RGB as well as alpha.