feat: add enableDirtyRepaints toggle for PR861 behaviour, default on

[wouterlucas]

Summary

Adds a construction-time enableDirtyRepaints setting (default true) that toggles the surgical dirty-quad repaint behaviour introduced in PR #861.

• true (default): PR perf: dirty quad buffer with surgical bufferSubData uploads #861 behaviour — permanent quad slots, dirty-ratio heuristic, surgical bufferSubData uploads, dedicated RTT buffer + renderRTT() split.
• false: opt-out legacy path — full quad buffer re-upload every frame via a single bufferData(STATIC_DRAW); all slot/dirty bookkeeping stays inert.

WebGL-only: CanvasRenderer always uses full uploads and is unaffected by this flag.

Changes

• src/main-api/Renderer.ts — new enableDirtyRepaints field on RendererMainSettings with JSDoc; resolved with ?? true and forwarded to Stage.
• src/core/Stage.ts — requestRenderListUpdate() only calls invalidateQuadBuffer() when enabled.
• src/core/CoreNode.ts — both isQuadDirty writes (visual-flag block in update(), texture setter) gated on the flag.
• src/core/renderers/webgl/WebGlRenderer.ts — cached readonly useDirtyRepaints flag (set once in the constructor); gates in addQuad, render, renderRTTNodes, invalidateQuadBuffer. Upload split into uploadFullBuffer (disabled path, no bookkeeping), uploadFullDirty (enabled full upload + bookkeeping reset) and uploadSurgicalQuads.

Tests

• Dirty-quad suites run enabled by default, matching the new default.
• Coverage for the disabled opt-out path: full upload with STATIC_DRAW, no bufferSubData, invalidateQuadBuffer no-op, RTT writes into the shared buffer; CoreNode stays clean and Stage skips invalidation when disabled.
• Verification: pnpm vitest run — 17 files / 292 tests pass; eslint 0 errors; prettier clean (tsc --build shows only the pre-existing happy-dom type error, also present on clean HEAD).

Refs: #861