Design doc for parent-child workflow Merkle linking:
- StartNodePayload.parentState: child → parent head state at spawn time
- StateNodePayload.childThread: parent → child final state hash
- Both also in refs[] for GC reachability
- 4-phase implementation plan
小橘 <xiaoju@shazhou.work>
- payload is source of truth with named fields (start, content, ancestors, compact)
- refs[] auto-derived by collectRefs(), pure GC index
- parent merged into ancestors[0]
小橘 <xiaoju@shazhou.work>