refactor: split into gameboy-rust workspace with core, demos, and desktop crates

Move the emulator into crates/gba-core, the original demo programs into
crates/gba-demos, and the minifb frontend into crates/desktop. The binary
is now gameboy-rust. Imports change from gba_rust to gba_core/gba_demos;
no behavior changes.

gba-core has no dependencies. Its integration tests use gba-demos as a
development dependency.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/gba-core/tests/exceptions.rs

@@ -0,0 +1,89 @@
+use gba_core::{
+    cpu::{Cpu, InstructionSet, Mode},
+    memory::{Memory, MemoryError, BIOS_SIZE, ROM_START},
+};
+use gba_demos::{demo_bios, exception_demo_program, EXCEPTION_DEMO_STEPS};
+
+#[test]
+fn original_demo_enters_supervisor_and_returns_to_arm_and_thumb() {
+    let mut memory = Memory::with_bios(exception_demo_program(), demo_bios()).unwrap();
+    let mut cpu = Cpu::new(ROM_START);
+    let expected = [
+        (8, Mode::Supervisor, InstructionSet::Arm),
+        (0x40, Mode::Supervisor, InstructionSet::Arm),
+        (0x44, Mode::Supervisor, InstructionSet::Arm),
+        (ROM_START + 4, Mode::System, InstructionSet::Arm),
+        (ROM_START + 8, Mode::System, InstructionSet::Arm),
+        (ROM_START + 12, Mode::System, InstructionSet::Thumb),
+        (8, Mode::Supervisor, InstructionSet::Arm),
+        (0x40, Mode::Supervisor, InstructionSet::Arm),
+        (0x44, Mode::Supervisor, InstructionSet::Arm),
+        (ROM_START + 14, Mode::System, InstructionSet::Thumb),
+        (ROM_START + 14, Mode::System, InstructionSet::Thumb),
+    ];
+    assert_eq!(expected.len(), EXCEPTION_DEMO_STEPS);
+    for (pc, mode, state) in expected {
+        cpu.step(&mut memory).unwrap();
+        assert_eq!(
+            (cpu.pc(), cpu.mode(), cpu.instruction_set()),
+            (pc, mode, state)
+        );
+    }
+    assert_eq!(cpu.registers()[10], 2);
+    assert_eq!(cpu.registers()[14], 0); // Supervisor LR did not replace System LR
+    assert_eq!(cpu.cpsr(), 0x3f);
+    for _ in 0..5 {
+        cpu.step(&mut memory).unwrap();
+    }
+    assert_eq!(cpu.pc(), ROM_START + 14);
+    assert_eq!(cpu.registers()[10], 2);
+}
+
+#[test]
+fn optional_bios_image_is_exactly_16_kib_read_only_and_not_mirrored() {
+    for size in [0, BIOS_SIZE - 1, BIOS_SIZE + 1] {
+        assert!(matches!(
+            Memory::with_bios(vec![], vec![0; size]),
+            Err(MemoryError::InvalidBiosSize(actual)) if actual == size
+        ));
+    }
+    let bios: Vec<u8> = (0..BIOS_SIZE).map(|index| index as u8).collect();
+    let mut memory = Memory::with_bios(vec![], bios).unwrap();
+    assert_eq!(memory.read32(0).unwrap(), 0x0302_0100);
+    assert_eq!(memory.read16(0x3ffe).unwrap(), 0xfffe);
+    assert_eq!(memory.read8(0x3fff).unwrap(), 255);
+    for address in [0x4000, 0x0100_0000] {
+        assert_eq!(memory.read8(address), Err(MemoryError::Unmapped(address)));
+        assert_eq!(
+            memory.write8(address, 1),
+            Err(MemoryError::Unmapped(address))
+        );
+    }
+    assert_eq!(memory.write8(0x3fff, 0), Err(MemoryError::ReadOnly(0x3fff)));
+    assert_eq!(
+        memory.write16(0x3ffe, 0),
+        Err(MemoryError::ReadOnly(0x3ffe))
+    );
+    assert_eq!(memory.write32(0, 0), Err(MemoryError::ReadOnly(0)));
+    assert_eq!(memory.read32(0).unwrap(), 0x0302_0100);
+    assert_eq!(memory.read16(0x3ffe).unwrap(), 0xfffe);
+}
+
+#[test]
+fn no_bios_is_mapped_by_default() {
+    let mut memory = Memory::new(vec![]).unwrap();
+    assert_eq!(memory.read32(0), Err(MemoryError::Unmapped(0)));
+    assert_eq!(memory.write32(0, 0), Err(MemoryError::Unmapped(0)));
+}
+
+#[test]
+fn reset_can_fetch_caller_supplied_vector_code() {
+    let mut bios = vec![0; BIOS_SIZE];
+    bios[0..4].copy_from_slice(&0xe3a0_002a_u32.to_le_bytes()); // MOV r0, #42
+    let mut memory = Memory::with_bios(vec![], bios).unwrap();
+    let mut cpu = Cpu::at_reset();
+    cpu.step(&mut memory).unwrap();
+    assert_eq!(cpu.registers()[0], 42);
+    assert_eq!(cpu.pc(), 4);
+    assert_eq!(cpu.cpsr(), 0xd3);
+}