ASS/PGS subtitles are now declared as regular cast text tracks; the custom
receiver draws the ones it can't render natively, keyed off the active track
id. Removes the custom message channel, the customData readback, and the
per-platform overlay-selection state so both senders treat every subtitle the
same.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WQ5PxMcMRmAMVxP8ZnYKnm