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/identities.py

@@ -173,6 +173,7 @@ class IdentitiesMixin(_MixinBase):
         ja3_hashes: Optional[str] = None,
         hassh_hashes: Optional[str] = None,
         tls_cert_sha256: Optional[str] = None,
+        kd_digraph_simhash: Optional[bytes] = None,
     ) -> None:
         statement = (
             update(AttackerIdentity)
@@ -181,6 +182,7 @@ class IdentitiesMixin(_MixinBase):
                 ja3_hashes=ja3_hashes,
                 hassh_hashes=hassh_hashes,
                 tls_cert_sha256=tls_cert_sha256,
+                kd_digraph_simhash=kd_digraph_simhash,
                 updated_at=datetime.now(timezone.utc),
             )
         )