Three new reference clusterers in fixture_harness:
* c2_callback_clusterer — union-find on overlapping C2 callback
sets across an attacker's sessions. Pass-clusterer for fixture 5
where two operators with distinct tooling share a C2 endpoint as
the campaign signal.
* shift_clusterer — deliberately-bad reference that buckets
attackers by majority session-start hour into night/day/swing.
Adversarial reference for fixture 5; proves operational schedule
is NOT a campaign signal.
* composite_signals_clusterer — union-find combining (ja3, hassh)
match OR overlapping C2 callback. Will serve as the pass-
clusterer for fixture 6 (noise_floor) where multiple campaigns
with heterogeneous signal types are scored together.
Also factored a small _union_find helper for the new clusterers
(existing time_window/credential_jaccard left untouched to avoid
mixing refactor with feature work).
Fixture 5 (multi_operator): one campaign, two operators with
distinct UKC roles. Actor A (broker, night shift): Delivery →
Exploitation → Persistence → C2. Actor B (post-ex, day shift):
Discovery → Lateral Movement → Collection → Exfiltration.
Distinct JA3/HASSH/ASN/IPs; shared C2 + payload hash.
Four tests: corpus shape (distinct fingerprints, shared C2,
disjoint shifts), pipeline pass via c2_callback_clusterer,
explicit harness sanity that fingerprint_clusterer cannot resolve
this fixture (documents which signal carries the campaign), and
adversarial shift_clusterer fragmentation.
Phase-handoff edges (the real load-bearing signal per the design
doc) wait for the production clusterer; this fixture will prove
they're needed when it ships.
