feat: let a profile say its pin is held on purpose

nethersx2-turnip-classic documents the shim rules of the revision it
pins, and says so in prose: the classic build is that revision. Compared
to HEAD it reported the same two changes every pass, and recaling would
have repointed its refs at rules this build never had.

pin_frozen says the freeze is deliberate, and the profile is then judged
against its own revision, exactly as one pinned to a superseded tag
already was. Recale and bump are refused for it, which is what makes the
declaration safe to trust.

tests/test_profile_sync.py

@@ -2160,6 +2160,43 @@ class TestWriteDryRun(unittest.TestCase):
         self.assertEqual(self.path.read_text(), before)
 
 
+class TestDeliberatelyFrozenPin(TestBuildReport):
+    """A pin held on purpose is judged against itself, like a frozen tag.
+
+    nethersx2-turnip-classic documents the shim rules of the revision it
+    pins and says so: recaling would repoint its refs at rules the classic
+    build never had. Without a way to say the freeze is deliberate, it
+    reports the same drift every pass and nobody can act on it.
+    """
+
+    def _frozen(self, reason="the classic build is this revision"):
+        profile = self._profile(["a.c:2"])
+        profile["source_commit"] = "pinned"
+        profile["pin_frozen"] = reason
+        return profile
+
+    def test_a_frozen_pin_is_checked_against_itself(self):
+        self.files[("pinned", "a.c")] = ["x", "the cited line"]
+        self.files[("headsha", "a.c")] = ["y"] * 40
+        report = build_report("test", self._frozen(), self.dir)
+        self.assertTrue(report.pinned_tag)
+        self.assertEqual(report.entries[0].parts[0].status, "ANCHORED")
+
+    def test_a_frozen_profile_is_never_recaled(self):
+        self.files[("pinned", "a.c")] = ["x", "the cited line"]
+        report = build_report("test", self._frozen(), self.dir)
+        path = Path(self.dir) / "p.yml"
+        path.write_text(SAMPLE, encoding="utf-8")
+        self.assertEqual(rebase_refs(path, report), [])
+        self.assertFalse(bump_commit(path, report))
+
+    def test_an_ordinary_profile_is_unaffected(self):
+        self.files[("pinsha", "a.c")] = ["x", "hit"]
+        self.files[("headsha", "a.c")] = ["pad", "x", "hit"]
+        report = build_report("test", self._profile(["a.c:2"]), self.dir)
+        self.assertFalse(report.pinned_tag)
+
+
 class TestProseSpacingIsNotADifference(unittest.TestCase):
     """A recale keeps the author's spacing; the check has to allow for it.