Error handling

The library exposes specific error classes:

• SemaphoreReleasedError for double releases;
• QueueClearedError when pending queue work is cleared;
• CircuitBreakerOpenError when execution is rejected by an open breaker;
• FlowCancelledError when pending debounce work is cancelled.

Catch these classes when the caller can recover from the flow-control decision. Let unexpected task errors propagate.