perf: dirty quad buffer with surgical bufferSubData uploads

[wouterlucas]

Description

Optimizes the per-frame quad upload in the WebGL renderer so that only the quads that actually changed are re-uploaded to the GPU, instead of uploading the entire quad buffer on every frame.

How it works

• Every main-scene node now owns a permanent slot (quadBufferIndex) in the quad buffer, assigned contiguously during the addQuad pass.
• When visual data changes (transforms, colors, alpha, texture), the node is marked dirty (isQuadDirty) in CoreNode.update().
• In render(), the renderer chooses between:
  • Surgical path: gl.bufferSubData per dirty node, copying each 20-float quad into a preallocated scratch buffer (no per-frame allocation).
  • Full upload: a single gl.bufferData when the render list changes structurally, the buffer grows, or more than 40% of the render list is dirty (N bufferSubData calls are more expensive than one bulk upload at that point).
• RTT renders use a dedicated CPU buffer and are unaffected by the main scene's slot bookkeeping; after an RTT pass the main buffer is force re-uploaded.
• Stage.requestRenderListUpdate() notifies the renderer (invalidateQuadBuffer()) on structural changes so slots are reset and everything is re-uploaded.

Changes

• src/core/CoreNode.ts — quadBufferIndex / isQuadDirty bookkeeping + dirty marking
• src/core/Stage.ts — expose render list; call renderer.invalidateQuadBuffer() on rebuilds
• src/core/platforms/GlContextWrapper.ts, WebGlContextWrapper.ts — arrayBufferSubData
• src/core/renderers/CoreRenderer.ts — optional invalidateQuadBuffer() hook
• src/core/renderers/webgl/WebGlRenderer.ts — dirty-ratio upload decision, surgical bufferSubData, dedicated RTT buffer
• Tests: CoreNode.test.ts, Stage.renderListUpdate.test.ts, WebGlRenderer.dirtyQuadBuffer.test.ts
• Visual regression: examples/tests/dirty-quad-buffer.ts + certified snapshots (covers move, color, alpha, RTT child move, add, remove)

Verification

• pnpm build passes
• pnpm vitest run — 509/509 tests pass
• pnpm lint clean

The merge-base changed after approval.

The merge-base changed after approval.

The merge-base changed after approval.