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.

tests/db/test_kd_digraph_rollup.py

@@ -0,0 +1,45 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+"""kd_digraph_simhash round-trips through update_identity_fingerprints
+and the campaign-clustering projection."""
+from __future__ import annotations
+
+from datetime import datetime, timezone
+
+import pytest
+
+from decnet.web.db.factory import get_repository
+
+
+@pytest.fixture
+async def repo(tmp_path):
+    r = get_repository(db_path=str(tmp_path / "kd.db"))
+    await r.initialize()
+    return r
+
+
+@pytest.mark.asyncio
+async def test_fingerprint_write_and_clustering_read(repo):
+    now = datetime.now(timezone.utc)
+    await repo.create_attacker_identity({
+        "uuid": "id-kd", "first_seen_at": now, "last_seen_at": now,
+    })
+    raw = bytes.fromhex("deadbeefcafef00d")
+    await repo.update_identity_fingerprints("id-kd", kd_digraph_simhash=raw)
+
+    rows = await repo.list_identities_for_clustering()
+    row = next(r for r in rows if r["uuid"] == "id-kd")
+    assert bytes(row["kd_digraph_simhash"]) == raw
+
+
+@pytest.mark.asyncio
+async def test_fingerprint_overwrite_to_none(repo):
+    now = datetime.now(timezone.utc)
+    await repo.create_attacker_identity({
+        "uuid": "id-kd2", "first_seen_at": now, "last_seen_at": now,
+    })
+    await repo.update_identity_fingerprints("id-kd2", kd_digraph_simhash=b"\x01" * 8)
+    # A later pass with no biometric clears it (full-overwrite contract).
+    await repo.update_identity_fingerprints("id-kd2", kd_digraph_simhash=None)
+    rows = await repo.list_identities_for_clustering()
+    row = next(r for r in rows if r["uuid"] == "id-kd2")
+    assert row["kd_digraph_simhash"] is None