async-state-safety

# Async State Safety ## Goal Prevent race regressions in async and stateful paths. ## Required Invariants - Define valid terminal states before coding. - Define allowed state transitions before coding. - Keep terminal transitions idempotent. - Ensure duplicate events cannot double-apply terminal effects. - Ensure retries do not corrupt state. ## Race Checklist - Cancellation racing completion - Timeout racing completion - Retry racing ack/receipt update - Concurrent updates to the same worker/channel record - Missing-handle and stale-handle behavior ## Implementation Checklist - Add or update transition guards. - Keep error handling explicit and structured. - Preserve status/event emission on all terminal branches. - Document why each race path converges safely. ## Verification Checklist - Run targeted tests for each touched race path. - Add at least one negative-path test for terminal convergence. - Add at least one idempotency test where applicable. - Run broad gate checks after targeted checks pass. ## Handoff Requirements - Terminal states and transition matrix - Race windows analyzed - Targeted commands and outcomes - Residual risks and follow-up tests