feat(clustering): combined edge weight + medium-tier wiring

The clusterer now drops a single high-tier function call in favor of
a tier-weighted sum. Tier multipliers (high=1.0, medium=0.6, low=0.2,
very_low=0.05) are tuned so the threshold (1.0) admits high-tier
agreement alone while leaving every weaker tier — and every
combination of weaker tiers — under threshold.

Per-tier discipline tested:
- high alone clusters
- medium alone does NOT cluster (supporting signal only)
- low alone does NOT cluster (fixture 1's failure mode)
- very-low alone does NOT cluster (fixture 2's failure mode)
- all three weak tiers stacked still don't reach threshold
- high + medium clusters (high already saturates)

The combination is forward-compatible: low + very-low contributions
are computed today but always project to 0.0 because the production
adapter doesn't populate credentials / ASN-edge inputs into the
fixture path yet. Their contribution becomes load-bearing in commit 7
when the low-tier landing tightens the F1 / F2 bounds.

Fixture 4 (paused_campaign) ratchet added: high-tier signal carries
the multi-day-silence campaign into one identity. Time-agnostic
invariant — silence is irrelevant to the edge weight.

decnet/clustering/impl/connected_components.py

@@ -34,8 +34,9 @@ from typing import Any, Iterable, Optional
 
 from decnet.clustering.base import Clusterer, ClusterResult
 from decnet.clustering.impl.similarity import (
+    EDGE_THRESHOLD,
     Observation,
-    high_weight_edge,
+    combined_edge_weight,
 )
 from decnet.logging import get_logger
 from decnet.web.db.repository import BaseRepository
@@ -43,13 +44,6 @@ from decnet.web.db.repository import BaseRepository
 log = get_logger("clustering.connected_components")
 
 
-# Threshold above which an edge survives into the graph. The high-tier
-# functions return 1.0 on agreement, so a literal >= 1.0 cutoff means
-# "exact match required." Once medium-tier edges combine, this becomes
-# a tunable.
-_EDGE_THRESHOLD = 1.0
-
-
 def cluster_observations(
     observations: Iterable[Observation],
 ) -> dict[str, str]:
@@ -81,7 +75,7 @@ def cluster_observations(
 
     for i, a in enumerate(obs_list):
         for b in obs_list[i + 1:]:
-            if high_weight_edge(a, b) >= _EDGE_THRESHOLD:
+            if combined_edge_weight(a, b) >= EDGE_THRESHOLD:
                 union(a.observation_id, b.observation_id)
 
     # Roots: each unique find(o) is a component representative. Use

decnet/clustering/impl/similarity.py

@@ -162,6 +162,63 @@ def very_low_weight_edge(a: Observation, b: Observation) -> float:
     return 1.0 if a.asn == b.asn else 0.0
 
 
+# ─── Combined weight ────────────────────────────────────────────────────────
+
+#: Tier multipliers applied to the per-tier edge scores when combining
+#: into a single weight. Tuned so that:
+#:
+#: * High-tier agreement alone (1.0) crosses the 1.0 threshold.
+#: * Medium-tier alone (max 1.0) yields 0.6 — below threshold.
+#: * Low-tier alone (max 1.0) yields 0.2 — defeats fixture 1's
+#:   credential-overlap-only failure mode.
+#: * Very-low alone (max 1.0) yields 0.05 — defeats fixture 2's
+#:   ASN-rotation failure mode.
+#:
+#: The ratio between tiers matters more than the absolute values: a
+#: tier should never combine its way past threshold without help from
+#: a stronger one.
+TIER_WEIGHTS = {
+    "high": 1.0,
+    "medium": 0.6,
+    "low": 0.2,
+    "very_low": 0.05,
+}
+
+#: Threshold a combined edge weight must meet to survive into the
+#: similarity graph. The connected-components impl drops anything
+#: under this before running union-find.
+EDGE_THRESHOLD = 1.0
+
+
+def combined_edge_weight(a: Observation, b: Observation) -> float:
+    """Sum of all four tier scores, weighted by :data:`TIER_WEIGHTS`.
+
+    Each per-tier function returns a score in ``[0, 1]``; the
+    weighted sum lets stronger tiers dominate without letting weaker
+    ones combine their way past threshold.
+
+    The connected-components clusterer compares this against
+    :data:`EDGE_THRESHOLD` to decide whether to draw an edge. Pure /
+    time-agnostic — fixture 7 forbids recency-decay weighting.
+
+    Commits 5–7 land each tier in the call site:
+
+    * Commit 5 (this commit): high + medium.
+    * Commit 6: + phase-handoff (a separate edge family, not a tier).
+    * Commit 7: + low + very_low.
+
+    Until commit 7 lands, the low / very_low contributions stay zero
+    by virtue of the underlying functions returning ``0.0`` whenever
+    their inputs are missing. The combination is forward-compatible.
+    """
+    return (
+        TIER_WEIGHTS["high"] * high_weight_edge(a, b)
+        + TIER_WEIGHTS["medium"] * medium_weight_edge(a, b)
+        + TIER_WEIGHTS["low"] * low_weight_edge(a, b)
+        + TIER_WEIGHTS["very_low"] * very_low_weight_edge(a, b)
+    )
+
+
 # ─── Adapter for the synthetic-corpus tests ─────────────────────────────────
 
 
@@ -206,5 +263,8 @@ __all__ = [
     "medium_weight_edge",
     "low_weight_edge",
     "very_low_weight_edge",
+    "combined_edge_weight",
     "from_synthetic",
+    "EDGE_THRESHOLD",
+    "TIER_WEIGHTS",
 ]