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/clustering/impl/__init__.py

@@ -0,0 +1,6 @@
+"""Concrete clusterer implementations.
+
+Each module here contains exactly one :class:`~decnet.clustering.base.Clusterer`
+subclass. New implementations register themselves in
+:func:`decnet.clustering.factory.get_clusterer`.
+"""