feat(clustering): similarity-graph primitives

Adds the four weight-tier edge functions as pure, time-agnostic
scoring primitives over an Observation projection. Each returns a
score in [0, 1]; the connected-components impl will combine + threshold
in subsequent commits.

Tier semantics (from IDENTITY_RESOLUTION.md):
- high   — JA3/HASSH/payload-hash/C2-endpoint exact match
- medium — phase-bucketed command-sequence Jaccard
- low    — credential-attempt-set Jaccard (defeated alone by F1)
- very low — ASN equality (defeated alone by F2)

Time-agnostic invariant is a static test: Observation has no time
fields, so no edge function can silently start using them. Fixture 7
forbids recency-decay clustering on multi-month APT campaigns.

A from_synthetic() adapter projects SyntheticAttacker corpora into
Observation; the production-row adapter lands when the clusterer
starts reading the attackers table.

tests/clustering/test_similarity.py

@@ -0,0 +1,221 @@
+"""Unit tests for the similarity-graph primitives.
+
+Each edge function is tested in isolation: agreement → high score,
+disagreement → zero, missing-data → zero. Combination logic +
+thresholds live in the connected-components impl and are covered by
+the fixture suite once those land.
+"""
+from __future__ import annotations
+
+import pytest
+
+from decnet.clustering.impl.similarity import (
+    Observation,
+    from_synthetic,
+    high_weight_edge,
+    low_weight_edge,
+    medium_weight_edge,
+    very_low_weight_edge,
+)
+
+
+def _obs(**kwargs) -> Observation:
+    """Build an Observation with sensible defaults for tests."""
+    kwargs.setdefault("observation_id", "obs-x")
+    return Observation(**kwargs)
+
+
+# ─── high_weight_edge ──────────────────────────────────────────────────────
+
+
+def test_high_weight_ja3_match():
+    a = _obs(ja3="ja3-stable")
+    b = _obs(ja3="ja3-stable")
+    assert high_weight_edge(a, b) == 1.0
+
+
+def test_high_weight_hassh_match():
+    a = _obs(hassh="hassh-stable")
+    b = _obs(hassh="hassh-stable")
+    assert high_weight_edge(a, b) == 1.0
+
+
+def test_high_weight_payload_hash_overlap():
+    a = _obs(payload_hashes=frozenset({"pl-1", "pl-2"}))
+    b = _obs(payload_hashes=frozenset({"pl-2", "pl-3"}))
+    assert high_weight_edge(a, b) == 1.0
+
+
+def test_high_weight_c2_overlap():
+    a = _obs(c2_endpoints=frozenset({"c2.example.com"}))
+    b = _obs(c2_endpoints=frozenset({"c2.example.com", "c2-alt.example.com"}))
+    assert high_weight_edge(a, b) == 1.0
+
+
+def test_high_weight_no_match():
+    a = _obs(ja3="ja3-a", hassh="hassh-a", payload_hashes=frozenset({"x"}))
+    b = _obs(ja3="ja3-b", hassh="hassh-b", payload_hashes=frozenset({"y"}))
+    assert high_weight_edge(a, b) == 0.0
+
+
+def test_high_weight_both_null_ja3_does_not_match():
+    """Both-null JA3 must not be treated as 'agreement' — that would
+    fuse every un-fingerprinted noise scanner into one mega-cluster."""
+    a = _obs(ja3=None, hassh=None)
+    b = _obs(ja3=None, hassh=None)
+    assert high_weight_edge(a, b) == 0.0
+
+
+def test_high_weight_empty_sets_no_match():
+    a = _obs(payload_hashes=frozenset(), c2_endpoints=frozenset())
+    b = _obs(payload_hashes=frozenset(), c2_endpoints=frozenset())
+    assert high_weight_edge(a, b) == 0.0
+
+
+# ─── medium_weight_edge ────────────────────────────────────────────────────
+
+
+def test_medium_weight_jaccard_full_match_in_one_phase():
+    a = _obs(commands_by_phase={"discovery": ("ls", "id", "uname -a")})
+    b = _obs(commands_by_phase={"discovery": ("ls", "id", "uname -a")})
+    assert medium_weight_edge(a, b) == pytest.approx(1.0)
+
+
+def test_medium_weight_jaccard_partial_match():
+    a = _obs(commands_by_phase={"discovery": ("ls", "id", "uname -a", "whoami")})
+    b = _obs(commands_by_phase={"discovery": ("ls", "id")})
+    # |A∩B|=2, |A∪B|=4 → 0.5
+    assert medium_weight_edge(a, b) == pytest.approx(0.5)
+
+
+def test_medium_weight_picks_max_across_phases():
+    a = _obs(commands_by_phase={
+        "discovery": ("ls",),
+        "exploitation": ("./payload", "chmod +x payload"),
+    })
+    b = _obs(commands_by_phase={
+        "discovery": ("ps",),  # 0.0
+        "exploitation": ("./payload", "chmod +x payload"),  # 1.0
+    })
+    assert medium_weight_edge(a, b) == pytest.approx(1.0)
+
+
+def test_medium_weight_no_shared_phase_returns_zero():
+    a = _obs(commands_by_phase={"discovery": ("ls",)})
+    b = _obs(commands_by_phase={"exploitation": ("./payload",)})
+    assert medium_weight_edge(a, b) == 0.0
+
+
+def test_medium_weight_disjoint_commands_in_shared_phase():
+    a = _obs(commands_by_phase={"discovery": ("ls",)})
+    b = _obs(commands_by_phase={"discovery": ("ps",)})
+    # |A∩B|=0, |A∪B|=2
+    assert medium_weight_edge(a, b) == 0.0
+
+
+def test_medium_weight_empty_corpora_returns_zero():
+    a = _obs()
+    b = _obs()
+    assert medium_weight_edge(a, b) == 0.0
+
+
+# ─── low_weight_edge ───────────────────────────────────────────────────────
+
+
+def test_low_weight_credential_jaccard_match():
+    a = _obs(credentials=frozenset({("root", "toor"), ("admin", "admin")}))
+    b = _obs(credentials=frozenset({("root", "toor"), ("admin", "admin")}))
+    assert low_weight_edge(a, b) == pytest.approx(1.0)
+
+
+def test_low_weight_credential_partial_overlap():
+    a = _obs(credentials=frozenset({("root", "toor"), ("admin", "admin")}))
+    b = _obs(credentials=frozenset({("root", "toor"), ("user", "user")}))
+    assert low_weight_edge(a, b) == pytest.approx(1 / 3)
+
+
+def test_low_weight_no_credentials_returns_zero():
+    a = _obs()
+    b = _obs(credentials=frozenset({("root", "toor")}))
+    assert low_weight_edge(a, b) == 0.0
+
+
+# ─── very_low_weight_edge ──────────────────────────────────────────────────
+
+
+def test_very_low_weight_asn_match():
+    a = _obs(asn=64500)
+    b = _obs(asn=64500)
+    assert very_low_weight_edge(a, b) == 1.0
+
+
+def test_very_low_weight_asn_mismatch():
+    a = _obs(asn=64500)
+    b = _obs(asn=64501)
+    assert very_low_weight_edge(a, b) == 0.0
+
+
+def test_very_low_weight_asn_null_returns_zero():
+    a = _obs(asn=None)
+    b = _obs(asn=64500)
+    assert very_low_weight_edge(a, b) == 0.0
+
+
+# ─── time-agnostic invariant ───────────────────────────────────────────────
+
+
+def test_observations_carry_no_timestamps():
+    """Compile-time guarantee: Observation has no time fields, so no
+    edge function can accidentally start using them. Fixture 7 forbids
+    recency-decay clustering."""
+    field_names = set(Observation.__dataclass_fields__.keys())
+    forbidden = {"first_seen", "last_seen", "started_at", "session_midpoint", "timestamp"}
+    assert field_names.isdisjoint(forbidden), (
+        f"Observation grew time fields: {field_names & forbidden}. "
+        "Fixture 7 (slow_burn) forbids recency-aware clustering."
+    )
+
+
+# ─── from_synthetic adapter ────────────────────────────────────────────────
+
+
+def test_from_synthetic_round_trip():
+    """The adapter projects a SyntheticAttacker into an Observation
+    that the edge functions can score over."""
+    from datetime import datetime, timezone
+    from tests.factories.campaign_factory import (
+        SyntheticAttacker, SyntheticSession,
+    )
+    from decnet.clustering.ukc import UKCPhase
+
+    now = datetime.now(timezone.utc)
+    sess = SyntheticSession(
+        session_id="s1",
+        attacker_id="a1",
+        decky_id="d1",
+        started_at=now,
+        duration_s=10.0,
+        phase=UKCPhase.DISCOVERY,
+        commands=["ls", "id"],
+        credentials_tried=[("root", "toor")],
+        payload_hash="pl-1",
+        c2_callback="c2.example.com",
+        truth_campaign_id="c1",
+        truth_actor_id="actor-1",
+    )
+    att = SyntheticAttacker(
+        attacker_id="a1", ip="1.1.1.1", asn=64500,
+        ja3="ja3-x", hassh="hassh-y",
+        first_seen=now, last_seen=now,
+        truth_campaign_id="c1", truth_actor_id="actor-1",
+        sessions=[sess],
+    )
+    obs = from_synthetic(att)
+    assert obs.observation_id == "a1"
+    assert obs.ja3 == "ja3-x"
+    assert obs.hassh == "hassh-y"
+    assert obs.asn == 64500
+    assert obs.payload_hashes == frozenset({"pl-1"})
+    assert obs.c2_endpoints == frozenset({"c2.example.com"})
+    assert obs.credentials == frozenset({("root", "toor")})
+    assert obs.commands_by_phase == {"discovery": ("ls", "id")}