Module pending_async 

PendingAsync + ExecutionState - engine’s async-suspension bookkeeping + execution liveness tracking per §5.3.

Structs

ExecutionState

Per-execution liveness tracker Stored in Engine.execution_state, keyed by ExecId. /// minimum-viable: just the output counter for GC bookkeeping; may extend.

PendingAsync

Bookkeeping for an Op suspended on a CommandId per docs/ENGINE.md §9.1. Stored in Engine.pending_async, keyed by CommandId.