feat(clustering): identity clusterer worker skeleton

Adds the decnet clusterer master-only command + provider-subpackage
shape (base.py + factory.py + impl/connected_components.py) so
subsequent commits can land similarity-graph features without
churning callers.

The skeleton ConnectedComponentsClusterer.tick is a no-op; the
worker shell is fully wired (bus consumer on attacker.observed +
attacker.scored, slow-tick fallback, health heartbeat, control
listener, ClusterResult fan-out to identity.formed/observation.linked
/merged). Subscribers on identity.> see no events from this clusterer
until edge functions land, but the lifecycle is in place.

decnet/cli/gating.py

@@ -29,7 +29,7 @@ MASTER_ONLY_COMMANDS: frozenset[str] = frozenset({
     "api", "swarmctl", "deploy", "redeploy", "teardown",
     "mutate", "listener", "profiler",
     "services", "distros", "correlate", "archetypes", "web",
-    "db-reset", "init", "webhook",
+    "db-reset", "init", "webhook", "clusterer",
 })
 MASTER_ONLY_GROUPS: frozenset[str] = frozenset({"swarm", "topology", "geoip"})