test: check site project links against MANIFEST in CI

tests/test_site.py

@@ -1,5 +1,8 @@
+import re
 from pathlib import Path
 
+from brand.build import projects as p
+
 _INDEX = Path(__file__).parent.parent / "docs" / "index.md"
 
 
@@ -12,3 +15,21 @@ def test_home_page_supplies_its_own_h1() -> None:
     decoration — and the site quietly grows a "Modern Python" heading above it.
     """
     assert "<h1" in _INDEX.read_text(encoding="utf-8")
+
+
+def test_every_listed_project_mark_has_a_site_entry() -> None:
+    """INVARIANT: the repos in the org site's project lists are exactly the repos in
+    `projects.py::MANIFEST` minus the `modern-di-*` integrations.
+
+    It is the site's half of the profile-row invariant. A hand-kept list of expected
+    repos drifts the same way the page does, so a repo added to the org and the profile
+    can still be missing here without anything failing.
+    """
+    listed = set(
+        re.findall(
+            r"^- \[`([^`]+)`\]\(https://github\.com/modern-python/\1\)",
+            _INDEX.read_text(encoding="utf-8"),
+            flags=re.M,
+        )
+    )
+    assert listed == {repo for repo in p.MANIFEST if not repo.startswith("modern-di-")}