perf(webgl): skip SDF buffer upload when text content is unchanged

[chiefcll]

What

Skips the per-frame bufferData upload of the shared SDF vertex buffer in render() when the buffer's content is guaranteed identical to what the GPU already holds.

Why

render() re-uploaded the entire used SDF buffer on every frame where any text existed — on a TV screen, that's every frame. A screen with a few hundred visible glyphs re-uploads tens-to-hundreds of KB at 60fps even when every text node is static, and bufferData (orphan-and-reallocate semantics) is the expensive call on the embedded driver stacks this engine targets. Any frame where images fade, focus rings animate, or colors pulse — but text holds still — paid this for nothing.

How

The dirty signal is derived inside the renderer, with no scene-graph invalidation hooks: the cache-hit text path (addSdfCachedQuads) writes byte-identical data at identical offsets as long as visit order and glyph counts are unchanged. A new sdfBufferChanged flag forces upload when any of these occur:

• a cache-miss write ran (addSdfQuads — layout/transform/color/alpha changed)
• the render list was rebuilt (invalidateQuadBuffer — write order changed)
• an RTT pass ran (RTT glyphs occupy the front of the shared buffer)
• the backing buffer grew (storage replaced)
• the used size differs from the last upload (contributor set changed — catches text nodes appearing/disappearing even when every individual write was a cache hit)

The decision is isolated in shouldUploadSdfBuffer() for unit testing. Context loss needs no handling: the engine's documented recovery is a full app reload (no webglcontextrestored path exists).

The per-frame memcpys and SdfRenderOp rebuilds are unchanged — only the GL transfer is elided, which is where the cost lives. Canvas2D backend untouched (no SDF path).

Testing

• New unit tests (WebGlRenderer.sdfUpload.test.ts): first-frame upload, skip on unchanged content + matching size, forced upload on cache-miss write / size mismatch, growth and render-list invalidation hooks (tested via Object.create(WebGlRenderer.prototype) — no GL context needed).
• pnpm build clean; webgl + CoreNode suites 83/83.
• Browser-verified in the examples app: static text-align renders crisp SDF paragraphs (skip path); stress-single-level-text animates text continuously with intact glyphs (cache-miss re-upload path).
• CI visual regression should be pixel-identical — the GPU receives the same bytes either way.

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