stops review + provisional LLM closure

Replay of 433b35d (server-26#170): traffic stops now open incidents
("45 Adam" stop, "CM2" stop), but the bridge MVA split 33/56 — one
transmission ("transport complete") was read as scene-resolved at 14:44
and an LLM closure was final, so the rest of the MVA opened a new one.

- LLM closure is now provisional (reopenable), like a timer close: it is
  inferred from a single transmission.
- review of 433b35d: backstop only matches a unit's own "on a stop"
  self-report (bare "car stop" mentions and "pull over" dropped), never on
  MTA/rail/bridge/fire/EMS/DPW talkgroups ("Train 4 holding on the stop"),
  negation looks 5 words back, and <=5-word reports ("Adam 3 on a stop")
  get a minimal scene instead of being skipped before the backstop.

c2-core: 471 pass.

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

drb-c2-core/tests/test_clearance_tracking.py

@@ -105,7 +105,7 @@ def test_traffic_stops_become_events():
     from app.internal.intelligence import _self_initiated_backstop as b
     for t in ("45 Adam on a stop, Eastbound Central Express.",
               "11-0. CM2 on the stop, southbound, KFLA on the right.",
-              "Car 7, traffic stop, Route 9 at Main"):
+              "Car 7 on a traffic stop, Route 9 at Main"):
         tags, typ, sev = b(t, [], None, "routine")
         assert "traffic-stop" in tags and typ == "police" and sev == "minor", t
     tags, typ, sev = b("Charlie 1. You put me out with a pedestrian on a parkway", [], None, "routine")
@@ -115,3 +115,28 @@ def test_traffic_stops_become_events():
     assert b("45-8, go ahead.", [], None, "routine") == ([], None, "routine")
     # an existing type/severity is never downgraded
     assert b("on a stop", ["dwi"], "police", "moderate") == (["dwi", "traffic-stop"], "police", "moderate")
+
+
+def test_stop_backstop_stays_off_rail_bridge_and_ems_channels():
+    from app.internal.intelligence import _self_initiated_backstop as b
+    none = ([], None, "routine")
+    assert b("Train 4 holding on the stop at Grand Central", [], None, "routine",
+             "MTA PD Districts 6/7/11 - Police Dispatch") == none
+    assert b("out with a disabled on the bridge, toll plaza", [], None, "routine",
+             "MTA Bridges and Tunnels - Whitestone/Throgs Neck Bridge Patrols") == none
+    assert b("Medic 2 pull over to the side and wait", [], None, "routine") == none
+    assert b("ran a plate for a car stop", [], None, "routine") == none
+    assert b("I dont think he is on a stop", [], None, "routine") == none
+    assert b("45 Adam on a stop", [], None, "routine", "Ch 1 (Patched with 155.310)")[0] == ["traffic-stop"]
+
+
+def test_short_stop_report_opens_a_scene():
+    import asyncio
+    from unittest.mock import patch
+    from app.internal import firestore as fstore, intelligence
+
+    async def run():
+        with patch.object(fstore, "doc_set"), patch.object(fstore, "doc_get_cached", return_value=None):
+            return await intelligence.extract_scenes("c1", "Adam 3 on a stop.", "Ch 1 (Patched with 155.310)")
+    scenes = asyncio.run(run())
+    assert len(scenes) == 1 and scenes[0]["tags"] == ["traffic-stop"] and scenes[0]["incident_type"] == "police"