test(ci): admit only named sibling-PR corpus consumers while they are pending

The strict corpus guard requires every listed consumer of the update-marker
corpus to exist. On this stacked PR the Electron reader lands in #132345 and
the two hand-off script readers in #132354, which merge before this PR, so
the guard went red here although the composite has all five.

Absence is now admitted only for consumers named in _LANDS_IN_SIBLING_PR
with the PR that lands them. A missing consumer no sibling lands, a fixture
whose every listed consumer is absent, and an allowance entry that names no
listed consumer all still fail. Once the siblings are on main the table can be
emptied and the full strict check applies unchanged.

(cherry picked from commit cfdfa6beeb4a1d8f17eb0ef7b078547b3c39cd5d)
(cherry picked from commit 3fb57d4a4e8805da89338ac0f469924171bcc0d1)

tests/ci/test_update_ci_routing.py

@@ -358,15 +358,31 @@ def _tracked_mentions(name: str) -> list[str]:
     return out.stdout.split()
 
 
+# Listed consumers that land in a named sibling PR merged BEFORE this one (the update batch
+# lands LOCK, COMMIT, WIN, DESK, SCRIPTS, POST, then this CI PR). Only these may be absent,
+# and only while the sibling is unmerged; once it lands, its files are in every tree this
+# guard sees. Empty this table when the siblings are on main: nothing else changes.
+_LANDS_IN_SIBLING_PR: dict[str, str] = {
+    "apps/desktop/electron/update-marker-corpus.test.ts": "#132345 desktop update gate",
+    "tests/scripts/desktop_update/test_desktop_update_posix_marker_corpus.py": "#132354 hand-off scripts",
+    "tests/scripts/desktop_update/test_desktop_update_windows_marker_corpus.py": "#132354 hand-off scripts",
+}
+
+
 def test_every_test_that_reads_a_shared_fixture_is_routed_by_it():
     """Derived, not remembered: every test file in the tree that names a shared fixture is
     a listed consumer, and the fixture selects each test lane that consumer's own edit would."""
     assert _CORPUS in cc._SHARED_FIXTURE_CONSUMERS
+    every_listed = {c for listed in cc._SHARED_FIXTURE_CONSUMERS.values() for c in listed}
+    stale = sorted(set(_LANDS_IN_SIBLING_PR) - every_listed)
+    assert not stale, f"sibling-PR allowance names no listed consumer: {stale}"
     for fixture, listed in cc._SHARED_FIXTURE_CONSUMERS.items():
         assert (_REPO / fixture).is_file(), f"missing shared fixture: {fixture}"
         assert listed, f"{fixture}: no listed consumers"
-        for consumer in listed:
-            assert (_REPO / consumer).is_file(), f"{fixture}: missing consumer: {consumer}"
+        absent = [c for c in listed if not (_REPO / c).is_file()]
+        unowned = [c for c in absent if c not in _LANDS_IN_SIBLING_PR]
+        assert not unowned, f"{fixture}: missing consumer(s) no sibling PR lands: {unowned}"
+        assert len(absent) < len(listed), f"{fixture}: every listed consumer is absent"
         readers = {p for p in _tracked_mentions(Path(fixture).name) if _TEST_FILE.search(p)}
         assert readers, f"{fixture}: no discovered test readers"
         assert readers <= set(listed), f"{fixture}: unlisted consumer(s) {sorted(readers - set(listed))}"
@@ -466,6 +482,35 @@ def test_shared_fixture_guard_rejects_phantom_graph(monkeypatch, tmp_path, missi
         test_every_test_that_reads_a_shared_fixture_is_routed_by_it()
 
 
+@pytest.mark.parametrize("case,error", [
+    ("sibling-absent", None),  # the stacked PR: one reader here, one landing in a sibling
+    ("unowned-absent", "no sibling PR lands"),
+    ("only-sibling", "every listed consumer is absent"),
+    ("stale-allowance", "names no listed consumer"),
+])
+def test_sibling_pr_allowance_admits_only_named_pending_consumers(monkeypatch, tmp_path, case, error):
+    fixture, here, pending = ("tests/fixtures/owned_corpus.json", "tests/test_owned_corpus.py",
+                              "tests/scripts/test_sibling_corpus.py")
+    (tmp_path / fixture).parent.mkdir(parents=True)
+    (tmp_path / fixture).write_text("{}", encoding="utf-8")
+    if case != "only-sibling":
+        (tmp_path / here).write_text("def test_reader(): pass\n", encoding="utf-8")
+    allowance = {} if case == "unowned-absent" else {pending: "#1 sibling"}
+    if case == "stale-allowance":
+        allowance["tests/test_removed_corpus.py"] = "#2 sibling"
+    monkeypatch.setattr(sys.modules[__name__], "_REPO", tmp_path)
+    monkeypatch.setattr(sys.modules[__name__], "_CORPUS", fixture)
+    monkeypatch.setattr(sys.modules[__name__], "_LANDS_IN_SIBLING_PR", allowance)
+    monkeypatch.setattr(cc, "_SHARED_FIXTURE_CONSUMERS",
+                        {fixture: (pending,) if case == "only-sibling" else (here, pending)})
+    monkeypatch.setattr(sys.modules[__name__], "_tracked_mentions", lambda _: [here])
+    if error is None:
+        test_every_test_that_reads_a_shared_fixture_is_routed_by_it()
+    else:
+        with pytest.raises(AssertionError, match=error):
+            test_every_test_that_reads_a_shared_fixture_is_routed_by_it()
+
+
 # -- ownership: the update entry points' real imports ---------------------------------------
 
 _SKIP_DIRS = {".git", ".venv", "venv", "node_modules", ".worktrees", "tests", "website", "__pycache__"}