S4a-3a of the PlexPlayer decomposition. Move two byte-equivalent
behaviours into lib/plexPlayerHooks.ts: usePlexProgress (the 10s
interval flush + pause/pagehide beacon + reopen cache-patch checkpoint;
absRef stays in the component since seek/skip/ended mutate it too) and
usePlayerVolume (HTML5 <video> 0..1 volume + independent mute, persisted
+ re-applied per session). usePlayerVolume is the seam the later R12
volume-consolidation sprint will rework to share playerVolume.ts's
tested pure core with the YouTube player. No behaviour change; verified
by self-E2E (volume-bar sets + persists video.volume, 'm' toggles mute
keeping the level, pause fires plexProgress 200).
peter
68f4834695