feat(correlation/attribution): substrate + idle handler (Phase 1)

v0 Phase 1 of ATTRIBUTION-ENGINE.md:

* AttributionStateRow SQLModel keyed on (identity_uuid, primitive)
  per ANTI direction — re-keying state rows when the v1 clusterer
  merges attackers is the migration debt v0 should not bake in.
  ATTRIBUTION-ENGINE.md updated with the deviation note.
* AttributionMixin: ensure_stub_identity_for_attacker, idempotent
  upsert_attribution_state, get_attribution_state[_for_identity],
  list_multi_actor_identities (the Phase 5 correlator's read).
* attribution.profile.{state_changed,multi_actor_suspected} bus
  topics + builder; wiki Service-Bus.md updated separately.
* attribution_worker.py: subscribes to attacker.observation.>,
  ensures stub identity per event, logs and continues. No merger,
  no state writes, no derived events — Phase 4 wires those.
* attribution/{aggregate.py,_thresholds.py} skeletons: Phase 2
  fills _aggregate_categorical, Phase 3 adds numeric+hash+dispatcher.

tests/bus/test_topics.py

@@ -87,3 +87,19 @@ def test_identity_builder() -> None:
 def test_identity_builder_rejects_empty() -> None:
     with pytest.raises(ValueError):
         topics.identity("")
+
+
+def test_attribution_builder() -> None:
+    assert (
+        topics.attribution(topics.ATTRIBUTION_PROFILE_STATE_CHANGED)
+        == "attribution.profile.state_changed"
+    )
+    assert (
+        topics.attribution(topics.ATTRIBUTION_PROFILE_MULTI_ACTOR_SUSPECTED)
+        == "attribution.profile.multi_actor_suspected"
+    )
+
+
+def test_attribution_builder_rejects_empty() -> None:
+    with pytest.raises(ValueError):
+        topics.attribution("")