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/dma.rs

@@ -686,10 +686,10 @@ fn invalid_memory_and_dma_register_destinations_are_diagnostics() {
         ),
         (
             SOURCE,
-            ROM_START,
+            0x0d00_0000,
             DmaError::Memory {
                 channel: 3,
-                error: MemoryError::ReadOnly(ROM_START),
+                error: MemoryError::ReadOnly(0x0d00_0000),
             },
         ),
         (