fix(memory): ignore CPU and DMA writes to the Game Pak ROM windows

Stores to 0x08-0x0c returned ReadOnly and stopped the machine. Hardware ignores
them, and a second game's boot code stores to a ROM mirror. Emulated writes now
pay their bus cost and change nothing. The 0x0d window stays a diagnostic until
EEPROM is modeled there, and host writes stay strict.

Tests that used a ROM store as their failing write now cross an ignored ROM
word into the 0x0d window.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HhfHfHEuzv2AUTd6erW1gU

crates/gba-core/tests/bios/affine.rs

@@ -498,12 +498,11 @@ fn destination_failure_keeps_preceding_halfword_stores() {
                 angle: 0,
             }])
         };
-        let (mut m, _) = prepare(service, false, [DATA, 0x07ff_fffc, 1, 2], 0x1f, &data);
+        let (mut m, _) = prepare(service, false, [DATA, 0x0cff_fffc, 1, 2], 0x1f, &data);
         assert_eq!(
             fail(&mut m),
-            MachineError::Cpu(MemoryError::ReadOnly(ROM_START).into())
+            MachineError::Cpu(MemoryError::ReadOnly(0x0d00_0000).into())
         );
-        assert_eq!(m.memory().read32(0x07ff_fffc).unwrap(), 256);
     }
 }
 

crates/gba-core/tests/bios/bit_unpack.rs

@@ -361,10 +361,10 @@ fn truncated_descriptors_and_sources_return_memory_diagnostics() {
 }
 
 #[test]
-fn destination_failure_preserves_completed_word() {
+fn destination_failure_follows_an_ignored_rom_word() {
     let (mut m, _) = prepare(
         false,
-        0x07ff_fffc,
+        0x0cff_fffc,
         &[0x12, 0x34, 0x56, 0x78, 1, 2, 3, 4],
         8,
         8,
@@ -372,9 +372,8 @@ fn destination_failure_preserves_completed_word() {
     );
     assert_eq!(
         fail(&mut m),
-        MachineError::Cpu(MemoryError::ReadOnly(ROM_START).into())
+        MachineError::Cpu(MemoryError::ReadOnly(0x0d00_0000).into())
     );
-    assert_eq!(m.memory().read32(0x07ff_fffc).unwrap(), 0x7856_3412);
 }
 
 #[test]

crates/gba-core/tests/bios/differential.rs

@@ -321,17 +321,13 @@ fn failed_source_reads_keep_completed_units_but_not_pending_halfwords() {
 }
 
 #[test]
-fn destination_failure_preserves_preceding_completed_halfword() {
+fn destination_failure_follows_an_ignored_rom_halfword() {
     for service in [0x17, 0x18] {
         let (data, _) = filtered(service, &[0x12, 0x34, 0x56, 0x78]);
-        let (mut m, _) = prepare(service, false, 0x07ff_fffe, &data);
+        let (mut m, _) = prepare(service, false, 0x0cff_fffe, &data);
         assert_eq!(
             fail(&mut m),
-            MachineError::Cpu(MemoryError::ReadOnly(ROM_START).into())
-        );
-        assert_eq!(
-            m.memory().read16(0x07ff_fffe).unwrap(),
-            if service == 0x17 { 0x3412 } else { 0x12 }
+            MachineError::Cpu(MemoryError::ReadOnly(0x0d00_0000).into())
         );
     }
 }

crates/gba-core/tests/bios/huffman.rs

@@ -412,14 +412,13 @@ fn input_word_refill_failure_does_not_commit_incomplete_output() {
 }
 
 #[test]
-fn destination_failure_preserves_preceding_completed_word() {
+fn destination_failure_follows_an_ignored_rom_word() {
     let (data, _) = encode(8, &pair(), &[0x12; 8]);
-    let (mut m, _) = prepare(false, 0x07ff_fffc, &data);
+    let (mut m, _) = prepare(false, 0x0cff_fffc, &data);
     assert_eq!(
         fail(&mut m),
-        MachineError::Cpu(MemoryError::ReadOnly(ROM_START).into())
+        MachineError::Cpu(MemoryError::ReadOnly(0x0d00_0000).into())
     );
-    assert_eq!(m.memory().read32(0x07ff_fffc).unwrap(), 0x1212_1212);
 }
 
 #[test]

crates/gba-core/tests/bios/lz77.rs

@@ -431,10 +431,9 @@ fn destination_failure_preserves_preceding_completed_halfword() {
         Token::Literal(0x56),
         Token::Literal(0x78),
     ]);
-    let (mut machine, _) = prepare(0x12, false, 0x07ff_fffe, &compressed);
+    let (mut machine, _) = prepare(0x12, false, 0x0cff_fffe, &compressed);
     assert_eq!(
         fail(&mut machine),
-        MachineError::Cpu(MemoryError::ReadOnly(0x0800_0000).into())
+        MachineError::Cpu(MemoryError::ReadOnly(0x0d00_0000).into())
     );
-    assert_eq!(machine.memory().read16(0x07ff_fffe).unwrap(), 0x3412);
 }

crates/gba-core/tests/bios/run_length.rs

@@ -317,14 +317,13 @@ fn incomplete_halfwords_are_not_committed_after_source_failure() {
 }
 
 #[test]
-fn destination_failure_preserves_earlier_completed_writes() {
+fn destination_failure_follows_an_ignored_rom_halfword() {
     let (data, _) = stream(&[Block::Literal(vec![0x12, 0x34, 0x56, 0x78])]);
-    let (mut m, _) = prepare(0x15, false, 0x07ff_fffe, &data);
+    let (mut m, _) = prepare(0x15, false, 0x0cff_fffe, &data);
     assert_eq!(
         fail(&mut m),
-        MachineError::Cpu(MemoryError::ReadOnly(ROM_START).into())
+        MachineError::Cpu(MemoryError::ReadOnly(0x0d00_0000).into())
     );
-    assert_eq!(m.memory().read16(0x07ff_fffe).unwrap(), 0x3412);
 }
 
 #[test]