[WebParagraph] Fix text pixelation, glyph clipping, and snapping offsets to avoid subpixel raster caching - #191736

Summary of Changes

This PR improves WebParagraph text rendering quality and caching on CanvasKit:

1. Fixes text blurriness / fractional distortion: Snaps physical screen offsets to whole integer device pixels and aligns the Canvas2D context translation to integer coordinates. This guarantees a 1:1 physical device pixel blit with FilterQuality.none.
2. Eliminates glyph clipping: Adds 2 physical pixels of safety antialiasing padding so glyph descenders/ascenders (e.g. g, j, y, _) are no longer clipped at the paint boundary.
3. Preserves fractional font sizes: Removes .floor() when generating CSS font strings (cssFontSize.toStringAsFixed(2)), allowing fractional font sizes to render accurately.
4. Simplifies caching (100% cache hit during scrolling): By snapping physical offsets to integer coordinates, text rendered on Canvas2D is always at phase (0.0, 0.0). As a result, the cached bitmap is valid across all translations and scrolling movements. All complex motion heuristics, frame sequence tracking, settle redraws, and 4-bin quantization state machines have been removed in favor of pure scale-based cache invalidation.

Fixes #188323