test(clustering): fixture 1 (shared_wordlist) + fixture-harness extraction

Two campaigns sharing a credential wordlist; everything else (ASN, IPs,
JA3, HASSH, active hours) divergent. Pass condition: clusterer must NOT
merge. Protects against the "credential overlap is identity" failure
mode that commodity wordlists invite.

* tests/clustering/fixture_harness.py — shared assert_fixture_bounds
  helper + identity_clusterer (placeholder, trivially correct on
  all-singleton fixtures) + credential_jaccard_clusterer (deliberately-
  bad reference used to PROVE the fixture catches what it should).
* tests/clustering/test_shared_wordlist_fixture.py — bounds pass with
  identity, bounds FAIL (homogeneity → 0) with the bad credential
  clusterer. The latter is the proof the fixture earns its keep.
* tests/fixtures/campaigns/shared_wordlist.{yaml,expected.yaml}.
* tests/clustering/test_lone_wolf_fixture.py — refactored onto the
  shared harness. No behavior change.

tests/fixtures/campaigns/shared_wordlist.expected.yaml

@@ -0,0 +1,21 @@
+# Bounds for fixture 1 (shared_wordlist).
+#
+# Ground truth: two distinct campaigns, one actor each → 2 truth-labels
+# of size 1. The clusterer must keep them separate. A correct algorithm
+# scores 1.0 across every metric on this fixture.
+#
+# Homogeneity is the load-bearing metric here: a clusterer that merges
+# the two campaigns based on shared credentials will tank homogeneity
+# (one predicted cluster contains members of two true campaigns).
+#
+# Bounds are loose at v1; tighten as the algorithm matures. Loosening
+# any bound to make CI pass requires PR-comment justification (per
+# CAMPAIGN_CLUSTERING.md §2).
+adjusted_rand_index:
+  min: 0.85
+homogeneity:
+  min: 0.90
+completeness:
+  min: 0.80
+singleton_recall:
+  min: 0.95