fix: the global gap report keeps standalone files

With no platform named, cross_reference passed no standalone cores, so every dual profile was laid out as libretro only. The CLI and the site's gaps page left out 133 files that Batocera and RetroBat run standalone and that no platform declares (eka2l1 90, dolphin 21, cemu 11). Both now pass the union of every platform's standalone cores.

tests/test_audit_regressions.py

@@ -5287,5 +5287,22 @@ class ReportsHoldTheDataTrees(unittest.TestCase):
                 ))
 
 
+class TheTreeKeepsStandaloneEntries(unittest.TestCase):
+    """With no platform named, the gap report laid every dual profile out as
+    libretro only: eka2l1, dolphin and xenia-edge lost 133 files that
+    Batocera and RetroBat run standalone and no platform declares."""
+
+    def test_a_standalone_only_entry_is_reported(self):
+        if not (ROOT / "database.json").exists():
+            self.skipTest("database.json is not built")
+        out = subprocess.run(
+            [sys.executable, "scripts/cross_reference.py", "--emulator", "eka2l1"],
+            cwd=ROOT, capture_output=True, text=True, timeout=600, check=True,
+        ).stdout
+        undeclared = re.search(r"(\d+) undeclared", out)
+        self.assertIsNotNone(undeclared, out[-500:])
+        self.assertGreater(int(undeclared.group(1)), 0)
+
+
 if __name__ == "__main__":
     unittest.main()