Match local sibling cleanup to upstream cancellation scope

When an upstream speech protocol cancels an entire playback context, abandoning one request must also stop local requests sharing that context. Deleting only the abandoned request map silently leaves siblings waiting for completion that remote cancellation may prevent. Mark the context interrupted, wake its local siblings, reject late admissions to that interrupted context, and send one bounded context cancellation. Preserve different contexts when cancellation succeeds; if cancellation cannot be delivered, discarding the shared socket necessarily fails its other active work. Request-level cancellation would permit finer isolation, so this rule depends on the protocol's actual scope.

Applies when: several local synthesis requests share a remotely cancellable playback context. Keywords: TTS websocket playback context cancellation sibling generator abandonment timeout protocol