correlation: rules decide, one cheap LLM call only when ambiguous; drop the consensus tiebreak (server-26#299)

The rules engine now commits every confident decision alone. _run_decision
marks a decision ambiguous (new_with_candidates, unfit_tg_candidate,
unverified_unit_link) and only then does upload._correlate make ONE
llm_correlator.decide call; a usable answer is committed, otherwise the rules
decision stands. Removed: _correlate_with_consensus, llm_correlator.tiebreak /
decisions_agree / tiebreak prompt, corr_smart_model, the correlation_smart
health tier, the LLM-orphan gate (_call_is_substanceless, effectively dead:
it could only fire on minor-severity untyped calls) and its sweep budget.
corr_consensus is now rules_only | llm | rules_fallback | rules_guard, with
corr_band naming why; admin debug and replay metrics read the new values.

Section fixes in the same pass:
- #115: no tiebreaker left to manufacture incidents; an LLM orphan of a
  rules "new" is honoured only for untyped, substanceless, routine/minor calls.
- #86: divergence veto bar moved to config (embedding_divergence_threshold);
  the band between it and the match bar is ambiguous, not a silent merge.
- #103: a unit link with no embedding is "unit_overlap_unverified" ->
  ambiguous, instead of skipping the veto and linking as verified.
- #189: unit-only and LLM links refused once an incident is open 90 min;
  unit links on incidents open >30 min are ambiguous; LLM links distance-
  guarded (incident_max_spread_km) and only to shown candidates.
- #190: severity steps down on an explicit stand-down phrase; new
  severity_peak keeps the monotonic max.
- #88: parse_llm_bool fails closed on "false"; scene closures honoured only
  on confident links or the incident the scene opened.
- #27: sweep budget is the scan window (wall clock), not 3 attempts.
- LLM prompt: "Reserve new" bullet no longer contradicts the #189 EXCEPT.
- updated_at never moves backward on a back-dated link.

Unmeasured: no live AI or replay (credits unfunded); band frequency, F1 and
thresholds are unverified. Must be re-scored on both answer keys in Replay
before merge.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-c2-core/tests/test_ai_health.py

@@ -136,15 +136,15 @@ async def test_transient_failure_alerts_once_threshold_crossed():
 async def test_recovery_alerts_once_after_an_alerted_episode():
     with patch.object(ai_health, "_post_webhook", new=AsyncMock()) as webhook:
         await ai_health.report_degraded(
-            "correlation_smart", "gemini", "gemini-1.5-pro",
+            "correlation_cheap", "gemini", "gemini-3.6-flash",
             "the Gemini account is out of credit", "top up billing", permanent=True,
         )
         webhook.reset_mock()
 
-        await ai_health.report_healthy("correlation_smart")
+        await ai_health.report_healthy("correlation_cheap")
 
     webhook.assert_awaited_once()
-    state = ai_health.snapshot()["correlation_smart"]
+    state = ai_health.snapshot()["correlation_cheap"]
     assert state["degraded"] is False
     assert state["alerted"] is False
     assert state["consecutive_failures"] == 0