feat(clustering): campaign-level similarity primitives

The signal taxonomy for the campaign clusterer (next commit). Mirror
of the identity-layer module but with edge families that don't
translate 1:1: phase-handoff (load-bearing for F5 multi_operator —
the signal the identity-side fingerprint-disagreement veto deliberately
isn't), shared-infra (vetoed at identity level, primary positive
signal here), temporal-overlap (pairwise-relative — F7 invariance
preserved), cohort (weak supporting weight only).

Tier weights tuned so phase-handoff alone crosses threshold (F5),
shared-infra + temporal-overlap together cross (canonical co-op
pattern), and shared-infra + cohort together do NOT (F1
shared_wordlist's failure mode). The F7 time-shift invariant is
explicitly tested on every time-bearing edge and on the combined
weight.

decnet/clustering/campaign/__init__.py

@@ -0,0 +1,5 @@
+"""Campaign clusterer — groups resolved identities into operations.
+
+The layer above identity resolution. See
+``development/CAMPAIGN_CLUSTERING.md`` for the signal taxonomy.
+"""