perf(export): measure 10-bit macOS decode before touching the software-decode predicate #676

Follow-up from #584 (HEVC 8-bit is decided — it keeps VideoToolbox, recorded in pipeline_macos.rs via #673).

10-bit H.264/HEVC on the macOS export path is still unmeasured and still excluded from the software-decode predicate by construction. Two things must happen before anyone touches it:

1. Measure it. Same protocol as docs(perf): record the HEVC 8-bit macOS decode measurements #673 (production Decoder, OPENSCREEN_MAC_DECODE, 1200 frames, best of three, machine at rest — Parsec disconnected), with the H.264 1080p control run first to validate the session against the recorded numbers. Lessons already learned there: VideoToolbox control figures move with machine state (212 fps on record vs 294–330 reproducible today), so treat the historical ×12.2 as an upper bound and re-run the witness on the day of the measurement.
2. Extend mac_frames::CpuFrames first. It converts decoded frames to NV12 8-bit — routing a 10-bit stream through the software path today would silently truncate the picture. The predicate change is only safe after CpuFrames renders a 10-bit format (P010 or equivalent).

Until both are done, 10-bit stays on VideoToolbox by the format == YUV420P condition in Decoder::open_with.