feat(clustering): roll session digraph SimHashes into identity centroid

The identity clusterer folds an identity's per-session
motor.digraph_simhash observations into one 8-byte bitwise-majority
centroid (denoises per-session jitter) and writes it to
AttackerIdentity.kd_digraph_simhash via update_identity_fingerprints —
the orphaned column is now populated. list_identities_for_clustering
projects it so the campaign clusterer can read it.

Extends the repo abstract + DummyRepo stub/coverage.

decnet/web/db/sqlmodel_repo/campaigns.py

@@ -105,6 +105,7 @@ class CampaignsMixin(_MixinBase):
             AttackerIdentity.hassh_hashes,
             AttackerIdentity.payload_simhashes,
             AttackerIdentity.c2_endpoints,
+            AttackerIdentity.kd_digraph_simhash,
         ).order_by(AttackerIdentity.created_at)
         if limit is not None:
             statement = statement.limit(limit)
@@ -129,6 +130,7 @@ class CampaignsMixin(_MixinBase):
                     "hassh_hashes": row.hassh_hashes,
                     "payload_simhashes": row.payload_simhashes,
                     "c2_endpoints": row.c2_endpoints,
+                    "kd_digraph_simhash": row.kd_digraph_simhash,
                 }
                 for row in result.all()
             ]