Replaces the imperative play/pause/seek/positionSync message soup with a
single host-authored PlaybackState (seq-ordered, anchor-extrapolated,
phase machine: loading/waitingForPeers/paused/playing) that doubles as
the heartbeat, plus guest status reports and host-applied control
requests. Fixes the guest seek-back loop while the host loads (readiness
was keyed on a pre-load !buffering snapshot and heartbeats broadcast
frozen positions), adds real group buffering coordination (stall grace,
scheduled simultaneous resumes, 15s safety timeout), rate-nudge drift
correction with passthrough-aware seek fallback, session-scoped message
handling (no lost messages during episode-switch detach gaps), and an
expected-state ledger replacing the racy remote-action flag.
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