SessionExecution fencing tests

Use deterministic production-path tests for SessionExecution fencing and recovery. Crash through session.resume(...) immediately after durable Step.Ended and Compaction.Ended commits, then prove the continue claim survives and its exact synthetic continuation runs once. Also prove replay settlement, atomic decline cancellation, conflicting-retry inertness, live-lease and newer-assistant exclusion, poison-row progress beyond one batch, and resumed stale-owner rejection before provider or local-tool dispatch.