- Add should_cancel callback parameter to BFS, DFS, and BestFirst strategies
- Add cancel() method for immediate cancellation (thread-safe)
- Add cancelled property to check cancellation status
- Add _check_cancellation() internal method supporting both sync/async callbacks
- Reset cancel event on strategy reuse for multiple crawls
- Include cancelled flag in state notifications via on_state_change
- Handle callback exceptions gracefully (fail-open, log warning)
- Add comprehensive test suite with 26 tests covering all edge cases
This enables external callers (e.g., cloud platforms) to stop a running
deep crawl mid-execution and retrieve partial results.
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