feat(mutator): bus-wake on decky mutate_request; adaptive sleep; heartbeat

The flat-fleet mutator was DB-poll-only and noisy — it logged
"no active deployment found" every 10s on idle hosts and ran
mutate_all at a fixed tick regardless of when the next decky
was due.

- mutate_all returns seconds-until-next-due; watch loop sleeps
  min(next_due, poll_interval_secs) with a 1s floor.
- "No deployment" is now idle, not an error: edge-triggered log
  on present<->absent transition instead of every tick.
- mutate_decky publishes decky.<name>.state on successful compose
  so UIs react in real time.
- New decky.*.mutate_request subscription lets API/CLI/UI force
  an immediate mutation of a specific decky without waiting for
  its interval; target name feeds mutate_all(only={...}).
- system.mutator.health heartbeat via run_health_heartbeat helper,
  bringing the mutator in line with DEBT-031 workers.

Tests: next_due return, only= filter, decky.<name>.state publish
on success, no publish on compose failure. Full mutator+topology-
mutator+bus suite (109) green.

decnet/bus/topics.py

@@ -50,6 +50,11 @@ TOPOLOGY_STATUS = "status"
 # Decky-level event types (second token).
 DECKY_STATE = "state"
 DECKY_TRAFFIC = "traffic"
+# On-demand mutation request — published by the API/CLI/UI, consumed by
+# the mutator's watch loop to force an immediate mutation of one decky
+# without waiting for its scheduled interval.  Underscored (not dotted)
+# to stay a single NATS token so the builder's validator accepts it.
+DECKY_MUTATE_REQUEST = "mutate_request"
 
 # Attacker event types (second token under the ``attacker`` root).  First
 # sighting, session boundary transitions, and score-threshold crossings