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.

decnet/web/db/models/__init__.py

@@ -59,6 +59,9 @@ from .attachments import (
 from .observations import (
     ObservationRow,
 )
+from .attribution_state import (
+    AttributionStateRow,
+)
 from .campaigns import (
     Campaign,
     CampaignsResponse,
@@ -252,6 +255,7 @@ __all__ = [
     "AttackerIdentity",
     "AttackerIntel",
     "AttackersResponse",
+    "AttributionStateRow",
     "ObservationRow",
     "ObservedAttachment",
     "SmtpTarget",