[bilipp]

Summary

Adds VideoSampleTap, exposed as PlayerSession.videoSamples, so a host can drive something from what is on screen. Lume uses it for Philips Hue light sync on tvOS (bilipp/Lume#374, which needs this merged first).

Implementation

• The video render pump offers each frame to the tap right after enqueueing it. The tap reads a sparse luma/chroma grid straight from the pixel buffer (read-only lock, never retained), converts it to linear light on BT.709 primaries (PQ/HLG scaled to SDR reference white, BT.2020/P3 converted), and keeps a short ring stamped with the frame PTS.
• Hosts read sample(atOrBefore: currentTime), since frames are enqueued ahead of the clock.
• Off by default; the render queue never waits on it, and an unreadable buffer is skipped. The ring empties on seek and on a newer serial.
• New fixtures quadrant.mp4 (8-bit H.264) and quadrant10.mp4 (10-bit HEVC) in generate-fixtures.sh.

Note: the commit message says the tap hands over the last displayed frame; it actually samples a colour grid as described above. Worth fixing in the squash message.

Testing

• FrameSampleTapTests (new) against both quadrant fixtures.
• Verified end to end on an Apple TV driving a Hue bridge from Lume.