Dispatch work item: all data needed to dispatch a node outside the mutex.

Pre-indexed edge lookups. Build once, use many times. Avoids O(edges) linear scan on every get_incoming/outgoing call.

Immutable per-execution data (never changes after creation).

Mutable per-execution data.

Full execution handle: immutable data in Arc, mutable data in Mutex. The Arc<ExecImmutable> can be cloned cheaply and borrowed while ExecMutable is lo