test: fixture-count floor assertion in conformance runner

Guard against the fail-open case where the runner discovers no (or too few)
shared fixtures and passes green having verified almost nothing. If the sibling
gcf fixture set is present but yields fewer than 150 files (mispathed/partial/
empty checkout), fail loudly. A wholly-absent checkout keeps its existing skip
(local ergonomics); in CI the separate gcf checkout step fails loudly if it
cannot clone. Current discovered count is 198.

Author: blackwell-systems

tests/test_conformance_v2.py

@@ -22,6 +22,22 @@ def _load_fixtures():
 
 FIXTURES = _load_fixtures()
 
+MIN_FIXTURES = 150
+
+
+def test_minimum_fixtures_discovered():
+    """Floor assertion: a green conformance run MUST have exercised the full shared
+    suite. The parametrized test below skips when FIXTURES is empty, so a mispathed or
+    partial checkout would otherwise pass having verified almost nothing. A wholly-absent
+    directory skips (local ergonomics); in CI the separate gcf checkout step fails loudly
+    if the repo cannot be cloned."""
+    if not FIXTURE_DIR.exists():
+        pytest.skip("conformance fixtures not found")
+    assert len(FIXTURES) >= MIN_FIXTURES, (
+        f"discovered only {len(FIXTURES)} conformance fixtures, expected at least "
+        f"{MIN_FIXTURES}; the shared gcf fixture set is incomplete or mispathed"
+    )
+
 
 def _json_norm(v):
     return json.loads(json.dumps(v))