Add a per-document HDR→SDR mapping applied at the final linear→sRGB encode, so
super-white (HDR) video highlights can be recovered instead of hard-clipped:
- core: HdrOutputMode {Clip (default), HighlightRolloff} on Document (serde
default), with a SetDocumentPropertiesAction variant for undoable edits.
- shaders: a fs_main_rolloff entry point (preview linear_to_srgb.wgsl + the export
inline shader) applying a C1 highlight knee — identity below 0.8, smooth rolloff
[0.8,∞)→[0.8,1). SDR below the knee is untouched; Clip stays the historical path.
- preview (stage.rs) and both export encodes (video_exporter.rs) pick the pipeline
variant from document.hdr_output_mode — one value per frame, so no per-pixel
uniform; mirrors the existing fs_main_straight pattern.
- UI: an "HDR output" dropdown in the Document section of the info panel.
Default (Clip) is bit-identical to previous behaviour. Completes Stage A:
HDR-correct input (pt 1) + SDR-safe output mapping. HDR export (10-bit P010/PQ)
and HDR display remain Stages B/C.
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