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-demos/src/lib.rs

@@ -0,0 +1,93 @@
+//! Original GBA test programs that drive `gba-core`. No game or Nintendo code.
+
+pub mod affine_demo;
+pub mod affine_raster_demo;
+pub mod bitmap_demo;
+pub mod effects_demo;
+pub mod graphics_demo;
+pub mod mosaic_demo;
+pub mod raster_demo;
+pub mod tile_demo;
+pub mod timer_demo;
+
+/// Steps needed to reach and execute the demo's final self-branch.
+pub const DEMO_STEPS: usize = 40;
+
+/// Steps needed to execute both software interrupts and the final self-branch.
+pub const EXCEPTION_DEMO_STEPS: usize = 11;
+
+/// Original test image, not a Nintendo BIOS or a BIOS-service replacement.
+/// The SWI vector branches to a handler that increments r10 and returns.
+pub fn demo_bios() -> Vec<u8> {
+    let mut bytes = vec![0; gba_core::memory::BIOS_SIZE];
+    bytes[8..12].copy_from_slice(&0xea00_000c_u32.to_le_bytes()); // B 0x40
+    bytes[0x40..0x44].copy_from_slice(&0xe28a_a001_u32.to_le_bytes()); // ADD r10, r10, #1
+    bytes[0x44..0x48].copy_from_slice(&0xe1b0_f00e_u32.to_le_bytes()); // MOVS pc, lr
+    bytes
+}
+
+/// Original ARM and Thumb software-interrupt test code.
+pub fn exception_demo_program() -> Vec<u8> {
+    let mut bytes: Vec<u8> = [
+        0xef00_0001_u32, // ARM SWI #1
+        0xe28f_0001,     // ADD r0, pc, #1 (Thumb offset 12, bit zero set)
+        0xe12f_ff10,     // BX r0
+    ]
+    .into_iter()
+    .flat_map(u32::to_le_bytes)
+    .collect();
+    bytes.extend(0xdf02_u16.to_le_bytes()); // Thumb SWI #2
+    bytes.extend(0xe7fe_u16.to_le_bytes()); // B .
+    bytes
+}
+
+/// Original ARM/Thumb, RAM, stack, and swap test code, not a Nintendo ROM.
+pub fn demo_program() -> Vec<u8> {
+    let mut bytes: Vec<u8> = [
+        0xe3a0_0003_u32, // MOV r0, #3
+        0xe3a0_1000,     // MOV r1, #0
+        0xe281_1001,     // loop: ADD r1, r1, #1
+        0xe250_0001,     // SUBS r0, r0, #1
+        0x1aff_fffc,     // BNE loop
+        0xe351_0003,     // CMP r1, #3
+        0x03a0_202a,     // MOVEQ r2, #42
+        0xe3a0_3402,     // MOV r3, #0x02000000 (work RAM)
+        0xe583_2000,     // STR r2, [r3]
+        0xe593_4000,     // LDR r4, [r3]
+        0xe3a0_d403,     // MOV sp, #0x03000000 (internal work RAM)
+        0xe28d_dc01,     // ADD sp, sp, #0x100
+        0xeb00_0000,     // BL subroutine at offset 56
+        0xea00_0005,     // B Thumb setup at offset 80
+        0xe92d_4010,     // subroutine: STMDB sp!, {r4, lr}
+        0xe1a0_5084,     // MOV r5, r4, LSL #1
+        0xe006_0594,     // MUL r6, r4, r5
+        0xe103_7096,     // SWP r7, r6, [r3] (old RAM value into r7)
+        0xe3a0_4000,     // MOV r4, #0 (demonstrate restoration from the stack)
+        0xe8bd_8010,     // LDMIA sp!, {r4, pc} (restore r4 and return)
+        0xe28f_800d,     // ADD r8, pc, #13 (Thumb entry at offset 100, bit 0 set)
+        0xe28f_9004,     // ADD r9, pc, #4 (ARM return at offset 96)
+        0xe12f_ff18,     // BX r8
+        0xe1a0_0000,     // NOP, skipped
+        0xeaff_fffe,     // ARM return: B .
+    ]
+    .into_iter()
+    .flat_map(u32::to_le_bytes)
+    .collect();
+    bytes.extend(
+        [
+            0x2006_u16, // Thumb entry: MOV r0, #6
+            0x2107,     // MOV r1, #7
+            0xf000,     // BL prefix
+            0xf803,     // BL suffix: target offset 114
+            0x6058,     // STR r0, [r3, #4]
+            0x282a,     // CMP r0, #42
+            0x4748,     // BX r9 (return to ARM)
+            0xb500,     // Thumb subroutine: PUSH {lr}
+            0x4348,     // MUL r0, r1
+            0xbd00,     // POP {pc} (stays in Thumb)
+        ]
+        .into_iter()
+        .flat_map(u16::to_le_bytes),
+    );
+    bytes
+}