feat: record the pin a prose reread confirmed

A prose citation whose text lived under several pins stayed in
read again until the text changed, so fixnes, zesarux and emucorex,
exact at their pins, would have refused every later bump.
prose_written_at names the pin a reader confirmed; it settles the
readings and never overrides a history with one candidate.

tests/test_profile_sync.py

@@ -2379,6 +2379,39 @@ class TestWriteDryRun(unittest.TestCase):
         self.assertEqual(readings, [[(repo, "pin1", "pin2")], []])
         self.assertTrue(ambiguous)
 
+    def test_a_confirmed_pin_settles_the_readings(self):
+        """fixnes: doOpenFDSBIOS lived under four pins and was exact at the
+        last; with nothing to record that reading, the bump was refused for
+        good. prose_written_at names the pin and the readings collapse."""
+        revisions = [
+            ("c2", {"source_commit": "pin2", "notes": "x"}),
+            ("c1", {"source_commit": "pin1", "notes": "x"}),
+        ]
+        repo = object()
+        readings, _ambiguous = profile_sync._writing_pairs(
+            {"source_commit": "pin2", "prose_written_at": "pin2"},
+            [("source", "", repo)], revisions, "c1",
+        )
+        self.assertEqual(readings, [[]])
+        later, _ = profile_sync._writing_pairs(
+            {"source_commit": "pin3", "prose_written_at": "pin2"},
+            [("source", "", repo)], revisions, "c1",
+        )
+        self.assertEqual(later, [[(repo, "pin2", "pin3")]])
+
+    def test_a_confirmed_pin_outside_the_history_changes_nothing(self):
+        revisions = [
+            ("c2", {"source_commit": "pin2", "notes": "x"}),
+            ("c1", {"source_commit": "pin1", "notes": "x"}),
+        ]
+        repo = object()
+        readings, ambiguous = profile_sync._writing_pairs(
+            {"source_commit": "pin2", "prose_written_at": "elsewhere"},
+            [("source", "", repo)], revisions, "c1",
+        )
+        self.assertEqual(readings, [[(repo, "pin1", "pin2")], []])
+        self.assertTrue(ambiguous)
+
     def test_a_pin_keyed_by_build_mode_is_read_for_its_mode(self):
         """eka2l1 pins a dict per build: read as a string, it gave no
         repository and its prose was never judged."""