perf(canvas): remove per-frame canvas reset and frame-loop allocations

[chiefcll]

Summary

Items 3 and 5 from the Canvas2D hot-path review (docs/canvas-renderer-review.md) — the per-frame tax every node pays on the canvas backend, which is the fallback path for the weakest target devices.

reset() no longer reallocates the canvas (item 3)

• The canvas.width = canvas.width reset trick forced the browser to tear down and reinitialize the canvas backing store every frame — one of the most expensive single operations in Canvas2D on embedded Blink. Replaced with setTransform(1,0,0,1,0,0) + clearRect, which is equivalent because the frame's save/restore pairs are balanced and globalAlpha is restored after each image draw.
• The clear-color fill was effectively always on: if (this.clearColor) is always true since normalizeCanvasColor(0x00000000) returns the non-empty string "rgba(0,0,0,0)" — so the default transparent clear color still paid a full-screen fillRect every frame. The fill is now gated on the color's alpha byte, tracked in a new clearColorAlpha field kept in sync by updateClearColor.

Frame-loop allocations killed (item 5)

• Path2D per clipped node in addQuad → beginPath() / rect() / clip(), allocation-free. In the standard TV layout every card sits under the viewport clip, so this was one allocation per node per frame.
• renderContext closure per shader node per frame → one preallocated arrow function field reading shaderContextNode / shaderContextTexture scratch fields, set before the shader's render() call and nulled after so a destroyed node or its texture isn't pinned.
• parseColor() object per non-white node → returns a module-level scratch object, documented as consume-synchronously-never-retain. The only caller chain (CanvasRenderer.renderContext → CanvasTexture.getImage) reads it synchronously and retains only the derived formatRgba string key.
• Rounded-clip Path2Ds in Rounded, RoundedWithShadow, and roundedRectWithBorder → beginPath on the context. The border's evenodd fill keeps its Path2D since fill(path, 'evenodd') needs a path object.

Deliberately out of scope

• Clip-run hoisting (the review's "better still": clip once per run of consecutive nodes sharing a rect) — a structural change to addQuad's save/restore model, better as its own PR.
• Review items 4 (tint-cache keying on premultiplied color) and 6 (smaller cleanups).

Tests

• 6 new CanvasRenderer unit tests: reset semantics (including asserting zero canvas.width writes via a setter spy), transparent-clear skip, opaque-clear fill, updateClearColor sync, allocation-free clipping calls, and shader-callback reference identity across nodes.
• New colorParser.test.ts covering the scratch-reuse contract (same object across calls, white singleton untouched).
• Full unit suite passes (326 tests); tsc --build clean.
• Per the review's success criteria the visual regression suite must stay pixel-identical — relying on CI's pnpm test:visual for that.

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