Player backends share static per-backend EventChannel names, so two
overlapping instances (episode handoff, quick exit/reopen) collide:
the newer instance's listen displaces the older sink, the older
instance's late cancel then tears down the newer stream — silently
freezing its events — and the final cancel earns the engine's
"No active stream to cancel" reply, which the framework can only
report as an uncatchable fatal FlutterError (363 events).
Track the owning instance per channel: only the current owner sends
the native cancel; a displaced instance drops its dead subscription.
Residual benign teardown races are dropped in beforeSend.
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