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/effects.rs

@@ -0,0 +1,61 @@
+use gba_core::{
+    io::*,
+    memory::{Memory, MemoryError, OAM_START as OAM, PALETTE_START as PAL, VRAM_START as VRAM},
+    video::{rgb555_to_rgb888 as rgb, Framebuffer, HEIGHT, WIDTH},
+};
+
+#[path = "effects/colors.rs"]
+mod colors;
+#[path = "effects/demo.rs"]
+mod demo;
+#[path = "effects/windows.rs"]
+mod windows;
+
+fn setup() -> Memory {
+    let mut m = Memory::new(vec![]).unwrap();
+    m.write16(DISPCNT, 0x700).unwrap();
+    m.write16(PAL, 0x4210).unwrap();
+    for (bg, color) in [0x001f, 0x03e0, 0x7c00].into_iter().enumerate() {
+        let bg = bg as u32;
+        m.write16(PAL + (bg + 1) * 2, color).unwrap();
+        m.write16(BG0CNT + bg * 2, (((24 + bg) << 8) | (bg << 2) | bg) as u16)
+            .unwrap();
+        for offset in (0..32).step_by(2) {
+            m.write16(VRAM + bg * 0x4000 + offset, (bg as u16 + 1) * 0x1111)
+                .unwrap();
+        }
+    }
+    for obj in 0..128 {
+        m.write16(OAM + obj * 8, 0x200).unwrap();
+    }
+    for offset in (0..32).step_by(2) {
+        m.write16(VRAM + 0x10000 + offset, 0x1111).unwrap();
+    }
+    m.write16(PAL + 0x202, 0x7fff).unwrap();
+    m
+}
+
+fn render(m: &Memory) -> Framebuffer {
+    let mut f = Framebuffer::default();
+    m.render_frame(&mut f).unwrap();
+    f
+}
+
+fn pixel(m: &Memory, x: usize, y: usize) -> u32 {
+    render(m).pixels()[y * WIDTH + x]
+}
+
+fn object(m: &mut Memory, index: u32, a: u16, b: u16, c: u16) {
+    m.write16(OAM + index * 8, a).unwrap();
+    m.write16(OAM + index * 8 + 2, b).unwrap();
+    m.write16(OAM + index * 8 + 4, c).unwrap();
+}
+
+fn window(m: &mut Memory, h: u16, v: u16, inside: u16, outside: u16) {
+    m.write16(DISPCNT, m.read16(DISPCNT).unwrap() | 0x2000)
+        .unwrap();
+    m.write16(WIN0H, h).unwrap();
+    m.write16(WIN0V, v).unwrap();
+    m.write16(WININ, inside).unwrap();
+    m.write16(WINOUT, outside).unwrap();
+}