fix: the US Mega-CD BIOS also serves Latin America

jgenesis picks it for GenesisRegion::Americas, md-emu for boot byte 0x7a, ares for an NTSC-U disc: one code for every disc of the Americas, Brazilian Tec Toy releases included. The three tagged it north-america alone, so --region brazil dropped the BIOS those discs need. genesis_plus_gx and picodrive already carried both.

emulators/ares.yml

@@ -170,7 +170,7 @@ files:
   # Also used by Mega CD 32X (desktop-ui/emulator/mega-cd-32x.cpp)
   - name: SegaCDBIOS9303.bin
     system: mega-cd
-    region: [north-america]
+    region: [north-america, latin-america]
     required: true
     sha256: "fb477cdbf94c84424c2feca4fe40656d85393fe7b7b401911b45ad2eb991258c"
     validation: [sha256]

emulators/jgenesis.yml

@@ -77,7 +77,7 @@ files:
   - name: "bios_CD_U.bin"
     system: sega-megacd
     description: "Sega CD BIOS (US)"
-    region: [north-america]
+    region: [north-america, latin-america]
     required: true
     size: 131072
     validation: [size]

emulators/md-emu.yml

@@ -94,7 +94,7 @@ notes: |
 files:
   - name: bios_CD_U.bin
     system: sega-megacd
-    region: [north-america]
+    region: [north-america, latin-america]
     required: true
     agnostic: true
     size: 131072

tests/test_region.py

@@ -1072,5 +1072,27 @@ class ANameNoCodeOpensCarriesNoRegion(unittest.TestCase):
         self.assertNotIn("region", entry)
 
 
+class MegaCdAmericasIsOneCode(unittest.TestCase):
+    """A Sega CD disc carries one region code for all the Americas, so every
+    emulator picking the US BIOS by that code picks it for a Brazilian disc
+    too. jgenesis, md-emu and ares tagged it north-america alone, and a
+    --region brazil pack dropped the BIOS those discs boot with."""
+
+    def test_a_us_mega_cd_bios_includes_latin_america(self):
+        import yaml
+
+        root = Path(__file__).resolve().parents[1]
+        narrow = []
+        for path in sorted((root / "emulators").glob("*.yml")):
+            profile = yaml.safe_load(path.read_text(encoding="utf-8")) or {}
+            for entry in profile.get("files") or []:
+                system = str(entry.get("system", ""))
+                regions = entry.get("region") or []
+                if system in ("sega-megacd", "mega-cd") and "north-america" in regions \
+                        and "latin-america" not in regions:
+                    narrow.append(f"{path.stem}: {entry.get('name')}")
+        self.assertEqual(narrow, [])
+
+
 if __name__ == "__main__":
     unittest.main()