Add fillRect(), strokeRect(), clearRect() drawing operations and
fillStyle/strokeStyle/lineWidth/globalAlpha property support to the
Canvas 2D API.

- CSS crate: add parse_color_string() for canvas color parsing (hex,
rgb/rgba, named colors, transparent) with hex digit validation
- DOM crate: add rasterization functions with affine transform support,
source-over alpha blending, axis-aligned fast path, and general
inverse-mapping path for rotated/skewed transforms
- JS crate: wire up fillRect/strokeRect/clearRect native methods,
add property setter interception for fillStyle/strokeStyle/lineWidth/
globalAlpha with spec-compliant validation (invalid colors silently
ignored, non-finite lineWidth rejected)
- Tests: color parsing (12 tests), rectangle rasterization with
transforms and alpha blending (8 tests), JS integration (14 tests)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Add Canvas2dState, Canvas2dContext, and AffineTransform to the dom crate,
with full state stack (save/restore) and CTM operations (translate, rotate,
scale, transform, setTransform, resetTransform, getTransform). Wire up JS
bindings on the CanvasRenderingContext2D wrapper object. 25 Rust-level and
10 JS integration tests covering all transform operations, nested
save/restore, edge cases, and matrix inversion.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Add HTMLCanvasElement support with backing pixel buffer, layout integration
as a replaced element, render pipeline integration, and JS bindings for
getContext('2d'), width/height properties.

- DOM: canvas backing buffer storage (init, resize, clear per spec)
- Layout: canvas intrinsic sizing (default 300x150, from width/height attrs)
- Render: canvas buffers composited as images via DrawImage pipeline
- JS: getContext('2d') returns CanvasRenderingContext2D with canvas back-ref
- JS: width/height getters/setters that resize and clear the buffer

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>