Guest switch dispatch pre-marked its dedup key and fired-and-forgot, so
any failure (fetch error, reload busy with an auto-advance, navigation
race with the host exiting) silently stranded the guest on the old media.
A CurrentPlaybackDispatcher now marks a key handled only after the sink
reports success against the committed identity, with a serialized
in-flight slot, timeout, and generation reset; the reconciler re-offers
unattached media on every host heartbeat, making the heartbeat the retry
channel. Fetches that outlive their dispatch are re-validated against the
current snapshot so a stale switch can't override the live one.
hostExitedPlayer now rides the controller's ordered message queue with
host authentication instead of racing state handling in the provider.
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