SessionExecution cleanup fallback fence

Trap: running coordinator cleanup as soon as interruption starts looks safe because the owner is stopping, but runner finalizers may still publish and a successor may begin against uncertain state. Fix: [[01a0aebe-9317-766d-8924-b86991fefef9]] must block successors until runner and finalizers settle; cleanup then requires the live exact claim, publishes/reconciles exactly one interruption, and clears only that claim—never successor ownership.