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/correlation/attribution/__init__.py

@@ -0,0 +1,21 @@
+"""DECNET attribution engine — v0 aggregation library.
+
+Pure library: per-(identity, primitive) state machine over BEHAVE-SHELL
+observations. No I/O, no bus, no DB. The bus subscriber and DB writes
+live in :mod:`decnet.correlation.attribution_worker` so this package
+stays trivially testable with synthetic observation lists.
+
+See ``development/ATTRIBUTION-ENGINE.md`` for the full design and the
+explicit bright line: this engine does NOT do persona classification
+(HUMAN/LLM/SCRIPTED), does NOT gate access, does NOT attribute to
+named persons. It surfaces *behavioural coherence* and *behavioural
+drift*, and stops there.
+"""
+from __future__ import annotations
+
+from decnet.correlation.attribution.aggregate import (
+    AttributionState,
+    aggregate_observations,
+)
+
+__all__ = ["AttributionState", "aggregate_observations"]