implement cooperative heartbeat-based locking for rolling deployments

[dembrane-sam-bot]

• What is this change?: Implements heartbeat-based file locking and cooperative standby for Cloud Run rolling deployments to prevent split-brain duplicates during startup/deployment.
• What did Sam notice that led to this?: Rolling updates spin up a new container while the old container is still draining. The new container's _recover_from_reactions immediately re-queued in-flight tasks (left at :eyes: or :hourglass: by the draining container), creating duplicate sessions and GCS write collisions.
• Tier?: Tier 3 (runtime)
• Confidence?: 100% (passes all 565 runtime tests + 6 brand new heartbeat-lock tests + 45 structural eval tests).