fix: key the casefold index on its source dict

tests/test_casefold_cache.py

@@ -0,0 +1,30 @@
+"""The case-insensitive name index answers for the database it was asked about."""
+
+from __future__ import annotations
+
+import sys
+import unittest
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+sys.path.insert(0, str(REPO_ROOT / "scripts"))
+
+from common import _casefold_name_index  # noqa: E402
+
+
+class CasefoldIndexFollowsItsSource(unittest.TestCase):
+    def test_a_new_dict_gets_its_own_index(self):
+        first = {"BIOS.ROM": ["a"]}
+        self.assertEqual(_casefold_name_index(first)["bios.rom"], ["a"])
+        second = {"Bios.Rom": ["b"]}
+        self.assertEqual(_casefold_name_index(second)["bios.rom"], ["b"])
+
+    def test_a_grown_dict_is_reindexed(self):
+        index = {"A.BIN": ["a"]}
+        _casefold_name_index(index)
+        index["B.BIN"] = ["b"]
+        self.assertIn("b.bin", _casefold_name_index(index))
+
+
+if __name__ == "__main__":
+    unittest.main()