Stop a back-dated call from silently disabling every recency gate (server-26#74)

_call_fits_incident measured incident idle with the signed helper while every
other recency gate in the file uses the unsigned one. On the re-correlation
sweep, `now` is the call's own started_at, which can precede the incident's
last activity, so the value went negative.

Negative idle made `idle_min >= 15` false, which meant the content-divergence
veto never ran and unit overlap was accepted unconditionally -- on a shared
dispatch backbone that is the feedback loop that lets one incident absorb a
whole talkgroup. It also made `idle_min < 20.0` true at any back-dating, so a
tactical channel returned tactical_default for every swept orphan out to the
90-minute bound.

One variable feeds all four gates in the function, so this is a one-line change
at the source. The signed value is untouched where it belongs: callers still
compute corr_incident_idle_min themselves, so debug output keeps its meaning.

Direction is toward more splitting, on the sweep path only, which is the point
-- the bug was suppressing an over-merge veto. Forward-dated calls and anything
inside the thresholds behave exactly as before.

Two tests added alongside the existing idle-gate cases; both fail on the old
line and pass on the new one. 266 pass, 0 fail.

Refs server-26#74, #5, #80.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

drb-c2-core/app/internal/incident_correlator.py

@@ -1598,7 +1598,15 @@ def _call_fits_incident(
     Thin calls (no units/vehicles/coords) never reach this function —
     they are intercepted before it in correlate_call.
     """
-    idle_min = _incident_idle_minutes(inc, now) if now is not None else 9999.0
+    # Gate comparisons in this function must use the unsigned distance, not the
+    # signed value: the re-correlation sweep anchors `now` to the call's own
+    # started_at, which can be earlier than the incident's last activity and
+    # send the signed value negative — silently defeating every `idle_min`
+    # gate below (content-divergence veto and the tactical default alike).
+    # See `_idle_gate_minutes` docstring. The signed value is reported to
+    # callers separately as `corr_incident_idle_min` (they compute it via
+    # `_incident_idle_minutes` themselves) — nothing here needs it.
+    idle_min = _idle_gate_minutes(inc, now) if now is not None else 9999.0
     inc_id   = inc.get("incident_id", "?")
 
     # ── 1. Unit overlap ───────────────────────────────────────────────────────