feat: hold an upstream pin a port vendors

upstream_frozen says why upstream_commit stays: the port vendors that
upstream tree, synced to that release, or merged that commit. The view
is then read at its own pin, the way pin_frozen holds source_commit, and
a bump leaves it. Outside a GitHub fork network nothing else could tell
aps3e's vendored RPCS3 1f080485 from an upstream read on a date.

schemas/emulator.schema.json

@@ -912,6 +912,10 @@
       "type": "string",
       "description": "Why source_commit is held on purpose rather than followed. The profile is then judged against its own revision, like one pinned to a superseded tag: comparing it to HEAD says nothing and recaling would repoint its refs at code the build it documents never had."
     },
+    "upstream_frozen": {
+      "type": "string",
+      "description": "Why upstream_commit is held on purpose rather than followed: the port vendors that upstream tree, synced to that release, or merged that commit. The upstream view is judged against its own revision and a bump leaves it, the way pin_frozen holds source_commit."
+    },
     "version_source": {
       "type": "object",
       "additionalProperties": false,

scripts/profile_sync.py

@@ -1136,7 +1136,20 @@ def select_views(
             continue
         seen.add((repo.host, repo.slug))
         views.append(RepoView(repo, pin, head, origin, field))
-    return _follow_fork_base(_follow_submodule(views, cache_dir, offline), cache_dir, offline)
+    views = _follow_fork_base(_follow_submodule(views, cache_dir, offline), cache_dir, offline)
+    return _hold_upstream(profile, views)
+
+
+def _hold_upstream(profile: dict, views: list[RepoView]) -> list[RepoView]:
+    """An upstream tree the port vendors or synced to is judged at that
+    revision: aps3e carries RPCS3 1f080485, yaps2 PCSX2 342db515."""
+    if not str(profile.get("upstream_frozen") or "").strip():
+        return views
+    return [
+        replace(view, head=view.pin, origin=f"{view.origin}, held")
+        if view.field == "upstream" else view
+        for view in views
+    ]
 
 
 def _follow_submodule(

tests/test_profile_sync.py

@@ -4912,6 +4912,20 @@ class UpstreamVendoredAsASubmodule(unittest.TestCase):
         self.assertEqual(views[1].head, "nesbox-tip")
 
 
+class AHeldUpstreamStaysOnItsPin(unittest.TestCase):
+    """aps3e vendors RPCS3 1f080485 and yaps2 PCSX2 342db515: a bump moved
+    upstream_commit to the upstream HEAD neither port carries."""
+
+    def test_the_upstream_view_is_read_at_its_pin(self):
+        views = [
+            profile_sync.RepoView("fork", "p", "h", "source_commit", "source"),
+            profile_sync.RepoView("up", "vendored", "tip", "upstream_commit", "upstream"),
+        ]
+        held = profile_sync._hold_upstream({"upstream_frozen": "vendored tree"}, views)
+        self.assertEqual((held[0].head, held[1].head), ("h", "vendored"))
+        self.assertEqual(profile_sync._hold_upstream({}, views), views)
+
+
 class RealignReadsThePathAsWritten(unittest.TestCase):
     """hatarib cites hatari/src/tos.c, a path of hatariB. When only
     upstream_commit moved, realign stripped the hatari/ prefix, read