[web] Unify ui.Canvas behind a shared EngineCanvas frontend - #196348

Unifies ui.Canvas across CanvasKit and Skwasm behind a shared EngineCanvas frontend (primitives/canvas.dart) and stateless BackendCanvas boundary (backend/canvas.dart), turning CkCanvas and SkwasmCanvas into thin native translators. Centralizing argument validation, PictureImageTracker image retention, the per-draw BackendPaint lifecycle, point/atlas typed-array flattening, superellipse path emulation, getLocalClipBounds, and WebParagraph dispatch brings all web renderers onto a single implementation with consistent runtime behavior.

Part of #175630

Behavioral Changes

• Skwasm and Web Impeller now validate Canvas arguments identically to CanvasKit and native dart:ui:
  • Before: Skwasm silently truncated, misread, or passed mismatched buffer lengths to native code when given a transform matrix without 16 entries, an odd-length drawRawPoints list, or mismatched drawAtlas/drawRawAtlas transform, rect, and color lengths.
  • After: Each throws the same ArgumentError and message as CanvasKit and native dart:ui, without mutating the save stack, transform, or tracked images.
• Consistent debug assertions across all web renderers:
  • Skwasm and Web Impeller now assert on NaN rects, rounded rects, rounded superellipses, and offsets (including clipRSuperellipse/drawRSuperellipse on CanvasKit as well).
  • Drawing a disposed Image, Picture, Paragraph, Vertices, or atlas image, or calling drawAtlas/drawRawAtlas with colors and a null blendMode, now throws AssertionError across all web renderers (previously missing, or throwing StateError for disposed pictures on CanvasKit and disposed vertices).
• drawAtlas(colors: []) on Skwasm now treats an empty color list as "no colors":
  • Before: Skwasm passed a zero-length buffer and native code read one uninitialized color per sprite.
  • After: Matches CanvasKit and native dart:ui by passing null when colors is empty.
• drawParagraph now supports WebParagraph on Skwasm:
  • Before: SkwasmCanvas.drawParagraph threw a TypeError when given a WebParagraph; only CanvasKit routed it to WebParagraph.paint.
  • After: EngineCanvas.drawParagraph dispatches WebParagraph on every renderer.

Where to focus

• lib/src/engine/primitives/canvas.dart: Ordering invariant in EngineCanvas—all validation and throwing delegate unwraps (backendPath, backendImage, EngineVertices.delegate, getBackendFilter) run before _tracker recording and createBackendPaint(), so rejected calls never retain images or leak a native paint. Image and atlas draws pass defaultBlurTileMode: ui.TileMode.clamp; all other draws use the default decal mode.
• lib/src/engine/canvaskit/canvas.dart and lib/src/engine/skwasm/skwasm_impl/canvas.dart: Stateless BackendCanvas translators preserving backend-specific conversions (CanvasKit degree conversion, Mitchell cubic sampling, and float nine-slice centers; Skwasm StackScope conversions, integer nine-slice centers, and drawAtlas cullRect).
• lib/src/engine/canvaskit/{picture_recorder,picture,surface}.dart and lib/src/engine/skwasm/skwasm_impl/picture.dart: Recorders and internal raster canvases now wrap their backend canvas in EngineCanvas, and LayerPicture exposes imageTracker so EngineCanvas.drawPicture merges retained image sources in the frontend.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant in-code documentation (doc comments with ///).
• If this PR introduces a new feature or capability, I created and linked a website documentation issue or PR in flutter/website (or verified none is needed).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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