fix: resolve bios case collisions

Delete duplicate lowercase variants (identical MD5) of 12 files
that prevented clean checkout on case-insensitive filesystems
(Windows, macOS).

Canonical chosen per upstream source code:
- Atari 2600 KV*.WAV: uppercase (stella2023 hardcode)
- Vectrex VEC_Minestorm.vec: lowercase (BizHawk hardcode)
- MSX MSX{,2,2EXT}.ROM: uppercase (fMSX hardcode, 6/7 platforms)
- Doom DOOM2.WAD: uppercase (matches dir convention)

Add aliases to stella2014 and clk profiles so resolution still
finds the canonical source. Correct retrodeck Vectrex casing to
match upstream. Merge Commodore/Vic-20 into VIC-20.

Add tests/test_no_case_collisions.py as a CI guard.

Closes #49

tests/test_no_case_collisions.py

@@ -0,0 +1,58 @@
+from __future__ import annotations
+
+import os
+import unittest
+from collections import defaultdict
+from pathlib import Path
+
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+BIOS_ROOT = REPO_ROOT / "bios"
+
+
+class TestNoCaseCollisions(unittest.TestCase):
+    """Guard against case-colliding paths in bios/.
+
+    On case-insensitive filesystems (Windows, macOS default), git can only
+    check out one path per casefold-equivalence class, silently corrupting
+    clones. Issue #33 and #49 both stemmed from this.
+
+    .variants/ subdirs are exempt: they intentionally hold genuine content
+    variants disambiguated by hash suffix (e.g., BIOS.ROM.910fae67).
+    """
+
+    def test_bios_has_no_case_colliding_paths(self) -> None:
+        if not BIOS_ROOT.is_dir():
+            self.skipTest("bios/ directory not present")
+
+        collisions: list[str] = []
+        for root, dirs, files in os.walk(BIOS_ROOT):
+            if ".variants" in Path(root).parts:
+                continue
+
+            dir_groups: dict[str, list[str]] = defaultdict(list)
+            for d in dirs:
+                dir_groups[d.casefold()].append(d)
+            for variants in dir_groups.values():
+                if len(variants) > 1:
+                    rel = Path(root).relative_to(REPO_ROOT)
+                    collisions.append(f"DIR  {rel}: {sorted(variants)}")
+
+            file_groups: dict[str, list[str]] = defaultdict(list)
+            for f in files:
+                file_groups[f.casefold()].append(f)
+            for variants in file_groups.values():
+                if len(variants) > 1:
+                    rel = Path(root).relative_to(REPO_ROOT)
+                    collisions.append(f"FILE {rel}: {sorted(variants)}")
+
+        self.assertEqual(
+            collisions,
+            [],
+            "Case-colliding paths in bios/ would break Windows/macOS clones:\n"
+            + "\n".join(collisions),
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()