A unit call-sign is not a place

"Post 1-2" reached the geocoder, resolved against its talkgroup anchor and
produced a confident pin in the right town for an event with no known
location — while sitting in the same incident's `units` list the whole time.
A plausible wrong pin is worse than no pin: nothing downstream can tell it
is wrong.

Extraction returns `location` and `units` from one pass, so a string in both
is a misclassification, not two facts. Drop it before the geocoder sees it.

Closes server-26#52.

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

drb-c2-core/tests/test_incident_identity.py

@@ -28,7 +28,7 @@ from unittest.mock import AsyncMock, patch
 
 from app.internal.incident_correlator import (
     _build_context, _create_incident, _update_incident,
-    _resolve_location_pair, _verified_pin, clean_location,
+    _resolve_location_pair, _verified_pin, clean_location, location_is_unit,
 )
 
 NOW = datetime(2026, 8, 20, 7, 25, 0, tzinfo=timezone.utc)
@@ -340,3 +340,35 @@ async def test_a_legacy_incidents_title_is_not_claimed_by_the_next_call():
                           incident_type="police", severity="routine")
     assert "title" not in updates
     assert updates["location"] == "80 Grasslands Road"
+
+
+# ── server-26#52: a unit call-sign must never become a map pin ────────────────
+#
+# "Post 1-2" passed clean_location (it has a word in it), geocoded against the
+# Ossining anchor and produced a confident pin in the right town for an event
+# with no known location. It was in the same incident's `units` all along.
+
+@pytest.mark.parametrize("location,units", [
+    ("Post 1-2", ["1-2", "Lincoln", "Post 1-2"]),   # the dump's actual incident
+    ("post 1-2", ["Post 1-2"]),                     # case-blind
+    ("Post 1-2.", ["Post 1-2"]),                    # punctuation-blind
+    ("Engine 4", ["Engine 4", "Ladder 1"]),
+])
+def test_location_matching_a_unit_is_rejected(location, units):
+    assert location_is_unit(location, units) is True
+
+
+@pytest.mark.parametrize("location,units", [
+    ("Water Street", ["1-2", "Post 1-2"]),          # a real place, same incident
+    ("South High", []),                             # no units extracted
+    ("Riverdale Station", ["Lincoln"]),
+    ("", ["Post 1-2"]),                             # nothing to compare
+    (None, ["Post 1-2"]),
+])
+def test_real_places_survive_the_unit_check(location, units):
+    assert location_is_unit(location, units) is False
+
+
+def test_unit_check_does_not_match_on_substrings():
+    """`1-2` is a unit; "1-2 Main Street" is an address that contains it."""
+    assert location_is_unit("1-2 Main Street", ["1-2"]) is False