fix(svg): DPR-aware rasterization, source-region crop, zero-copy upload

[chiefcll]

Summary

Three related fixes to SVG loading in src/core/lib/textureSvg.ts:

• Sharpness on HiDPI / 4K — rasterize at stage.pixelRatio instead of logical (CSS) pixels. Previously the backing canvas was sized to the requested logical w × h, then the renderer upscaled it on a 2×-DPR display, giving a half-resolution result.
• Correct source-region crop — sx/sy/sw/sh now use the 9-arg drawImage(img, sx, sy, sw, sh, 0, 0, w, h) form, matching the source-rect semantics documented on ImageTextureProps. The old path drew at full destination size then called getImageData(sx, sy, sw, sh), which was a destination-region crop (top-left slice), not a sample of the intended source region.
• Zero-copy GPU upload — when createImageBitmap is available, the rasterized canvas is converted to an ImageBitmap and returned. The existing WebGL uploader in WebGlCtxTexture.ts:182-196 already accepts ImageBitmap, so the previous getImageData() CPU readback and the w·h·4-byte Uint8ClampedArray allocation are eliminated. ImageData is retained as the Chrome <50 fallback (preserves the Chrome 38 floor per BROWSERS.md).

Also sets img.crossOrigin = 'anonymous' for non-base64 SVG sources so cross-origin SVGs don't taint the canvas and trigger SecurityError on the readback path.

Why

• Memory: for a 512×512 SVG, removes a ~1 MB Uint8ClampedArray allocation per load (4 MB at 2× DPR). Larger SVGs scale linearly.
• CPU: removes the synchronous getImageData() readback in the common path.
• Correctness: the previous crop math silently produced wrong output when sw/sh < w/h. No callers currently exercise that path in-tree, but the prop docs advertise source-crop semantics.
• Resolution: users on 4K TVs (the engine's target environment) get sharp vector graphics by default instead of bilinear-upscaled rasters.

Touched files

• src/core/lib/textureSvg.ts — loadSvg rewritten (now async, accepts pixelRatio, returns ImageBitmap when available).
• src/core/CoreTextureManager.ts — added a public pixelRatio getter that forwards to stage.pixelRatio (so ImageTexture doesn't need to reach through private stage).
• src/core/textures/ImageTexture.ts — passes this.txManager.pixelRatio to loadSvg; collapsed the duplicate type === 'svg' / isSvgImage(src) branches.

Backwards compatibility

• Public API unchanged. loadSvg is module-local (re-exported only inside the engine), and its new pixelRatio parameter has a single caller in this repo.
• Texture-size semantics change: a caller passing w: 200, h: 200 on a 2× DPR display now gets a 400×400 GPU texture (still drawn at 200×200 logical). This matches how the engine treats other texture sources at HiDPI and is the whole point of the fix — flagging it here in case any consumer was relying on the prior behavior to keep memory low.
• premultiplyAlpha: false preserved (canvas-2D output is straight alpha).

Test plan

• pnpm build — clean
• pnpm test — 193/193 pass
• pnpm lint on touched files — no new warnings
• Manual: load an SVG in the examples app on a 2× DPR display, confirm sharpness
• Manual: load a cross-origin SVG (with permissive CORS headers), confirm it no longer throws SecurityError
• Manual: use sx/sy/sw/sh on an SVG texture, confirm the cropped output matches the documented source-region semantics
• Consider a visual-regression snapshot for an SVG texture under examples/tests/ to lock in the new behavior

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