fix: let the fallback mapping reach the largest file cited

openbor.c is 55k lines at the pin and 57k at HEAD, over a ceiling of
40k, so its ref reported CHANGED with nothing to act on: the mapping was
never attempted. Diffing that pair takes nine seconds, and the fallback
only runs once exact anchoring has failed, which is rare.

The ceiling now sits above it and still stops a pathological pair from
stalling a sweep of every profile. savesettings moved from 2675 to 2940
and the range maps cleanly.

tests/test_profile_sync.py

@@ -2160,6 +2160,34 @@ class TestWriteDryRun(unittest.TestCase):
         self.assertEqual(self.path.read_text(), before)
 
 
+class TestLargeFileCeiling(unittest.TestCase):
+    """The fallback mapping bails above a ceiling, and it sits high enough.
+
+    openbor.c is the largest file any profile cites, 55k lines against 57k
+    at HEAD, and the ref that mattered mapped only once the ceiling cleared
+    it. The guard stays so a pathological pair cannot stall a sweep.
+    """
+
+    def test_a_pair_over_the_ceiling_is_not_diffed(self):
+        big = ["x"] * (profile_sync.MAX_MATCH_LINES + 1)
+        result = profile_sync._map_changed(big, big, 1, 2)
+        self.assertEqual(result.status, "CHANGED")
+        self.assertIn("over", result.reason)
+
+    def test_a_pair_under_the_ceiling_is_mapped(self):
+        pin = ["a", "subject", "b"]
+        head = ["pad", "pad", "a", "subject", "b"]
+        result = profile_sync._map_changed(pin, head, 2, 2)
+        self.assertIsNotNone(result.start, "a small pair has to be diffed")
+        self.assertNotIn("over", result.reason or "")
+
+    def test_the_ceiling_clears_the_largest_file_any_profile_cites(self):
+        self.assertGreater(
+            profile_sync.MAX_MATCH_LINES, 57000,
+            "openbor.c is 57k lines at HEAD and its ref has to map",
+        )
+
+
 class TestRebaseWaitsForThePin(unittest.TestCase):
     """Recaling refs the pin cannot follow manufactures the desync.