feat(engine): add HDR video output pipeline

[vanceingalls]

Summary

Adds the ability to render HDR video output (H.265 10-bit, BT.2020) from HyperFrames compositions. When the renderer detects HDR source video, it automatically switches to the HDR output pipeline — no flags needed.

What it does

• Auto-detection — Probes each video source with ffprobe. If any has bt2020/PQ/HLG color metadata, the output switches to H.265 10-bit with correct color tags. SDR-only compositions are unaffected (H.264, bt709).
• HLG pass-through — Native HLG pixels from FFmpeg extraction are piped directly to the encoder without conversion. This avoids brightness loss from HLG→linear→PQ conversion (which requires an OOTF system gamma we can't reliably apply).
• Encoder HDR support — Both chunk and streaming encoders accept HDR presets: libx265, yuv420p10le, BT.2020 color primaries, hvc1 codec tag (required for Apple playback).
• WebGPU HDR capture (gated) — A complete WebGPU float16 readback pipeline is implemented and tested but gated behind headed Chrome (headless doesn't expose WebGPU). Ready for future use with WebGPU canvas content.
• HDR utilities — detectTransfer() (PQ vs HLG), getHdrEncoderColorParams(), analyzeCompositionHdr(). 15 unit tests.

Key design decisions

Decision	Why
No --hdr flag	SDR content encoded as HDR causes orange shift in browsers. Auto-detect eliminates this.
HLG pass-through (not HLG→PQ)	Conversion loses brightness without OOTF. Pass-through matches source exactly.
hvc1 codec tag	Apple QuickTime requires hvc1 (not hev1) for HEVC playback.
1-hour streaming timeout	HDR capture at ~6fps needs more time than the default 10-minute FFmpeg timeout.

Files changed

File	What changed
packages/engine/src/utils/hdr.ts	NEW — HDR detection, transfer types, encoder params (15 tests)
packages/engine/src/services/hdrCapture.ts	NEW — WebGPU readback, HLG conversion, PQ encode
packages/engine/src/services/streamingEncoder.ts	HDR presets, raw rgb48le input, color tags
packages/engine/src/services/chunkEncoder.ts	HDR presets, conditional color tags
packages/producer/src/services/renderOrchestrator.ts	Auto-detection loop, HDR pass-through capture path

How to test

Render a composition with an HDR video source. The output should be H.265 10-bit with HDR metadata visible in ffprobe (bt2020, arib-std-b67 or smpte2084). Plays correctly in QuickTime and on HDR displays.

Stack position

2 of 6 — Stacked on #258 (SDR/HDR normalization). Provides the encoder infrastructure that phases 1-5 build on.

🤖 Generated with Claude Code

[vanceingalls]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(hdr): shader transitions between HDR scenes + SDR rendering fixes #268
• feat(hdr): GSAP transforms and border-radius masks on HDR video #290
• feat(hdr): z-ordered multi-layer compositing with PQ support #289
• feat(engine): add HDR two-pass compositing — DOM layer + native HLG video #288
• feat(engine): add HDR video output pipeline #265 👈 (View in Graphite)
• fix(engine): normalize mixed HDR/SDR video inputs #258
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.