Features:
- fetch_work_item returns ExecutionState (Running/Terminal/Missing)
- renew_work_item_lock returns ExecutionState for cancellation detection
- ack_work_item accepts Option<WorkItem> for terminal orchestrations

Changes:
- Updated to duroxide 0.1.7 from crates.io
- 9 new cancellation validation tests
- 5 new long-polling validation tests
- 86 total validation tests passing

See CHANGELOG.md for full details.