* feat(hdr): add z-ordered multi-layer compositing with PQ support
Per-frame z-order analysis groups elements into DOM and HDR layers,
composited bottom-to-top. Adjacent DOM elements merge into single
screenshots. PQ (HDR10/smpte2084) support via sRGB-to-PQ LUT with
203-nit SDR reference white. queryElementStacking walks DOM for
effective z-index, groupIntoLayers splits on HDR/DOM boundaries.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
* fix(hdr): address review feedback across stack
- Document groupIntoLayers tie-break (V8 stable sort → DOM order).
- Expand layerCompositor docstring: merge rationale, visibility inclusion.
- Add tests: empty input, negative z-index, stable tie-break at equal z.
- Document getEffectiveZIndex CSS stacking-context limitations.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
