Add optional resume_state and on_state_change parameters to all deep
crawl strategies (BFS, DFS, Best-First) for cloud deployment crash
recovery.
Features:
- resume_state: Pass saved state to resume from checkpoint
- on_state_change: Async callback fired after each URL for real-time
state persistence to external storage (Redis, DB, etc.)
- export_state(): Get last captured state manually
- Zero overhead when features are disabled (None defaults)
State includes visited URLs, pending queue/stack, depths, and
pages_crawled count. All state is JSON-serializable.
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