fix: give the bizhawk DSi NAND slots their region

DecideNAND (MelonDS.cs:684-706) keeps, for a DSi title, only the NAND
slots whose bit is set in header byte 0x1B0. The six DSi_Nand entries
now carry that territory, so --region keeps one 240 MB image, not six.
AUS answers oceania, as every other Australian slot does.

emulators/bizhawk.yml

@@ -154,8 +154,9 @@ files:
     size: 251658304
     storage: large_file
     description: "DSi NAND (Japan)"
+    region: [japan]
     source_ref: "src/BizHawk.Emulation.Common/Database/FirmwareDatabase.cs:405, src/BizHawk.Emulation.Cores/Consoles/Nintendo/NDS/MelonDS.cs:484, 684-706"
-    note: "Slot NDS+NAND_JPN has a placeholder hash, so the scan never finds it. Only a user specification reaches it. Read in DSi mode. The size includes the 64-byte nocash footer."
+    note: "Slot NDS+NAND_JPN has a placeholder hash, so the scan never finds it. Only a user specification reaches it. Read in DSi mode. A title whose ID upper word is 0x000300xx (DSi) gets only the NAND of a region its header byte 0x1B0 allows, any other title the first held in the order JPN, USA, EUR, AUS, CHN, KOR. The size includes the 64-byte nocash footer."
 
   - name: "DSi_Nand_EUR.bin"
     system: nintendo-ds
@@ -163,8 +164,9 @@ files:
     size: 251658304
     storage: large_file
     description: "DSi NAND (Europe)"
+    region: [europe]
     source_ref: "src/BizHawk.Emulation.Common/Database/FirmwareDatabase.cs:406, src/BizHawk.Emulation.Cores/Consoles/Nintendo/NDS/MelonDS.cs:484, 684-706"
-    note: "Slot NDS+NAND_EUR has a placeholder hash, so the scan never finds it. Only a user specification reaches it. Read in DSi mode. The size includes the 64-byte nocash footer."
+    note: "Slot NDS+NAND_EUR has a placeholder hash, so the scan never finds it. Only a user specification reaches it. Read in DSi mode. A title whose ID upper word is 0x000300xx (DSi) gets only the NAND of a region its header byte 0x1B0 allows, any other title the first held in the order JPN, USA, EUR, AUS, CHN, KOR. The size includes the 64-byte nocash footer."
 
   - name: "DSi_Nand_USA.bin"
     system: nintendo-ds
@@ -172,8 +174,9 @@ files:
     size: 251658304
     storage: large_file
     description: "DSi NAND (USA)"
+    region: [north-america]
     source_ref: "src/BizHawk.Emulation.Common/Database/FirmwareDatabase.cs:407, src/BizHawk.Emulation.Cores/Consoles/Nintendo/NDS/MelonDS.cs:484, 684-706"
-    note: "Slot NDS+NAND_USA has a placeholder hash, so the scan never finds it. Only a user specification reaches it. Read in DSi mode. The size includes the 64-byte nocash footer."
+    note: "Slot NDS+NAND_USA has a placeholder hash, so the scan never finds it. Only a user specification reaches it. Read in DSi mode. A title whose ID upper word is 0x000300xx (DSi) gets only the NAND of a region its header byte 0x1B0 allows, any other title the first held in the order JPN, USA, EUR, AUS, CHN, KOR. The size includes the 64-byte nocash footer."
 
   - name: "DSi_Nand_AUS.bin"
     system: nintendo-ds
@@ -181,8 +184,9 @@ files:
     size: 251658304
     storage: large_file
     description: "DSi NAND (Australia)"
+    region: [oceania]
     source_ref: "src/BizHawk.Emulation.Common/Database/FirmwareDatabase.cs:408, src/BizHawk.Emulation.Cores/Consoles/Nintendo/NDS/MelonDS.cs:484, 684-706"
-    note: "Slot NDS+NAND_AUS has a placeholder hash, so the scan never finds it. Only a user specification reaches it. Read in DSi mode. The size includes the 64-byte nocash footer."
+    note: "Slot NDS+NAND_AUS has a placeholder hash, so the scan never finds it. Only a user specification reaches it. Read in DSi mode. A title whose ID upper word is 0x000300xx (DSi) gets only the NAND of a region its header byte 0x1B0 allows, any other title the first held in the order JPN, USA, EUR, AUS, CHN, KOR. The size includes the 64-byte nocash footer."
 
   - name: "DSi_Nand_CHN.bin"
     system: nintendo-ds
@@ -190,8 +194,9 @@ files:
     size: 251658304
     storage: large_file
     description: "DSi NAND (China)"
+    region: [china]
     source_ref: "src/BizHawk.Emulation.Common/Database/FirmwareDatabase.cs:409, src/BizHawk.Emulation.Cores/Consoles/Nintendo/NDS/MelonDS.cs:484, 684-706"
-    note: "Slot NDS+NAND_CHN has a placeholder hash, so the scan never finds it. Only a user specification reaches it. Read in DSi mode. The size includes the 64-byte nocash footer."
+    note: "Slot NDS+NAND_CHN has a placeholder hash, so the scan never finds it. Only a user specification reaches it. Read in DSi mode. A title whose ID upper word is 0x000300xx (DSi) gets only the NAND of a region its header byte 0x1B0 allows, any other title the first held in the order JPN, USA, EUR, AUS, CHN, KOR. The size includes the 64-byte nocash footer."
 
   - name: "DSi_Nand_KOR.bin"
     system: nintendo-ds
@@ -199,8 +204,9 @@ files:
     size: 251658304
     storage: large_file
     description: "DSi NAND (Korea)"
+    region: [south-korea]
     source_ref: "src/BizHawk.Emulation.Common/Database/FirmwareDatabase.cs:410, src/BizHawk.Emulation.Cores/Consoles/Nintendo/NDS/MelonDS.cs:484, 684-706"
-    note: "Slot NDS+NAND_KOR has a placeholder hash, so the scan never finds it. Only a user specification reaches it. Read in DSi mode. The size includes the 64-byte nocash footer."
+    note: "Slot NDS+NAND_KOR has a placeholder hash, so the scan never finds it. Only a user specification reaches it. Read in DSi mode. A title whose ID upper word is 0x000300xx (DSi) gets only the NAND of a region its header byte 0x1B0 allows, any other title the first held in the order JPN, USA, EUR, AUS, CHN, KOR. The size includes the 64-byte nocash footer."
 
   - name: "32X_G_BIOS.BIN"
     system: sega-32x

tests/test_region.py

@@ -916,6 +916,9 @@ class RegionalRequestsKeepWhatTheCodeLoads(unittest.TestCase):
         ("duckstation", ["brazil", "japan"], "scph5501.bin"),
         ("duckstation", ["brazil", "japan"], "scph7001.bin"),
         ("duckstation", ["brazil", "japan"], "ps2_scph39001.bin"),
+        ("bizhawk", ["north-america"], "DSi_Nand_USA.bin"),
+        ("bizhawk", ["oceania"], "DSi_Nand_AUS.bin"),
+        ("bizhawk", ["china", "japan"], "DSi_Nand_CHN.bin"),
     )
 
     def test_each_request_keeps_its_file(self):
@@ -929,6 +932,40 @@ class RegionalRequestsKeepWhatTheCodeLoads(unittest.TestCase):
 
 
 
+class DsiNandFollowsTheHeaderRegion(unittest.TestCase):
+    """BizHawk's DecideNAND keeps, for a DSi title, only the NAND slots whose
+    bit is set in header byte 0x1B0 (MelonDS.cs:684-706): JPN, USA, EUR, AUS,
+    CHN, KOR. The six DSi_Nand_<REGION>.bin entries carried no region, so a
+    --region request kept all six 240 MB images."""
+
+    SLOTS = {
+        "JPN": ["japan"],
+        "USA": ["north-america"],
+        "EUR": ["europe"],
+        "AUS": ["oceania"],
+        "CHN": ["china"],
+        "KOR": ["south-korea"],
+    }
+
+    def _profile(self) -> dict:
+        return common.load_emulator_profiles(
+            os.path.join(os.path.dirname(__file__), "..", "emulators")
+        )["bizhawk"]
+
+    def test_each_slot_carries_its_territory(self):
+        files = {f["name"]: f for f in self._profile()["files"]}
+        for slot, expected in self.SLOTS.items():
+            with self.subTest(slot=slot):
+                declared = files[f"DSi_Nand_{slot}.bin"].get("region")
+                self.assertEqual(declared, expected)
+                self.assertTrue(set(declared) <= region.REGIONS)
+
+    def test_a_request_keeps_its_nand_and_withdraws_the_others(self):
+        nands = {f"DSi_Nand_{slot}.bin" for slot in self.SLOTS}
+        drops = emulator_region_drops([("bizhawk", self._profile())], False, ["north-america"])
+        self.assertEqual(nands & drops, nands - {"DSi_Nand_USA.bin"})
+
+
 class FontRomsCarryNoTerritory(unittest.TestCase):
     """A CJK font ROM is a language, not a territory. blueMSX tagged KANJI.rom
     and MSXKANJI.rom [japan], which a --region south-korea request withdrew