feat(memory): add region-dependent Thumb open-bus snapshots

Repeat the mapped PC+4 halfword for Thumb code in EWRAM, palette RAM, VRAM, and ROM. Use the aligned PC+4 word for BIOS/OAM. Keep retained BIOS history separate from current unused-memory reads. Reuse one instruction snapshot without extra data cycles or host/fetch/DMA fallbacks.

Keep IWRAM and 16 MiB region-crossing fetches diagnostic until prior bus history is modeled. Add original regressions for regions, alignments, mirrors, physical wrap, signed and unaligned loads, block/stack/PC loads, aliases, modes, short ROMs, strict errors, BIOS isolation, and nominal device progression. Record documented rules and remaining limits.

Darwin arm64 validation: 890 workspace tests and 821 core/demo tests in each of debug and release; 39 preparation tests; native ROM windows in both profiles; ten window/graphics smoke modes; fmt, clippy, and rustdoc. Public ARM, Thumb, memory, and BIOS reports remain identical to previous passing results and between profiles. No physical-hardware or independent public Thumb open-bus conformance claim.

crates/gba-core/src/cpu.rs

@@ -32,6 +32,8 @@ mod tests;
 #[cfg(test)]
 mod thumb_empty_tests;
 #[cfg(test)]
+mod thumb_open_bus_tests;
+#[cfg(test)]
 mod thumb_tests;
 #[cfg(test)]
 mod timing_tests;