Dolby Vision P5/P8 (native dynamic metadata via VideoToolbox) #12

No Dolby Vision handling exists in the Swift layer. FFmpeg's DV RPU decoder is compiled (CONFIG_DOVI_RPUDEC 1) but unused; DV streams currently play as their HDR10/HLG base layer at best.

Design (PLAN.md §6): preserve dvcC config + RPU side data into CMSampleBuffer format-description extensions so VideoToolbox tone-maps natively.

Risk (PLAN.md §7): P5 (IPT-PQ-C2, no BL fallback) is hard — phase-gated spike testing dvh1/dvhe signaling through AVSampleBufferVideoRenderer on-device; if VT won't tone-map P5, fall back to Metal tone mapping or document as limitation. P7 dual-layer (single-layer display) is not currently committed — decide scope here.