4K HDR + transparent WebM

Highlights

• HDR10 MP4 — render 10-bit HDR10 with BT.2020 color and SMPTE ST 2084 PQ transfer. Pass --hdr to the CLI.
• Transparent WebM — alpha-channel WebM via VP9 yuva420p for downstream compositors. Add --alpha and emit .webm.
• 4K — confirmed throughput at 3840×2160 on a single H100 (≈40 fps headless Chromium + GPU encode).

Breaking

None.

Fixes

• Subpixel jitter on transform-driven animations when willChange was set on the parent.
• Audio drift on long renders (>2h) caused by clock-drift in the rAF shim.