Agent heartbeats now carry an applied-topology snapshot. The master
heartbeat handler compares the reported version_hash against what
canonical_hash yields for the hydrated topology pinned to that host
and flags Topology.needs_resync on divergence (or when the agent
reports no topology at all while master expects one).
The mutator watch loop gains reconcile_agent_resyncs, which re-pushes
the current hydrated blob via AgentClient.apply_topology without
touching status, then clears the flag on success. Push failures leave
the flag set so the next tick retries.
New decnet.agent.heartbeat asyncio loop wired into the agent FastAPI
lifespan. Every 30 s the worker POSTs executor.status() to the master's
/swarm/heartbeat with its DECNET_HOST_UUID for self-identity; the
existing agent mTLS bundle provides the client cert the master pins
against SwarmHost.client_cert_fingerprint.
start() is a silent no-op when identity env (HOST_UUID, MASTER_HOST) is
unset or the worker bundle is missing, so dev runs and un-enrolled hosts
don't crash the agent app. On non-204 responses the loop logs loudly but
keeps ticking — an operator may re-enrol mid-session, and fail-closed
pinning shouldn't be self-silencing.
