Covers all three layers of the salvaged fix:
1. Adapter-side cancellation: /stop, /new, /reset cancel the in-flight
adapter task, release the guard, and let follow-up messages through;
/new keeps the guard installed until the runner response lands, then
drains the queued follow-up in order.
2. Adapter-side self-heal: a split-brain guard (done owner task, lock
still live) is healed on the next inbound message and the user gets
a reply instead of being trapped in infinite busy acks. A guard
with no recorded owner task is NOT auto-healed (protects fixtures
that install guards directly).
3. Runner-side generation guard: stale async runs whose generation was
bumped by /stop or /new cannot clear a newer run's _running_agents
slot on the way out.
11 tests, all green.
Refs #11016
