Chunk Lifecycle

How run_chunks.status protects worker ownership and prevents stale workers from completing or releasing chunks they no longer own.

Notes

Run creation inserts chunks as pending.
Coordinator dispatch marks selected pending chunks with dispatched_at as it enqueues bounded run.chunk.ready windows.
Workers can claim a chunk only when the owning run is running.
Completion and release both compare the stored lease timestamp to the worker's claimed lease.
Cancellation changes open chunks to cancelled.