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/scanlines.rs

@@ -0,0 +1,38 @@
+use gba_core::{
+    cpu::Cpu,
+    display::{CYCLES_PER_FRAME, CYCLES_PER_LINE, HBLANK_START, VBLANK_START},
+    dma::DMA_BASE,
+    input::{Button, Buttons},
+    io::*,
+    machine::{Machine, StepKind},
+    memory::{Memory, OAM_START as OAM, PALETTE_START as PAL, ROM_START, VRAM_START as VRAM},
+    video::{rgb555_to_rgb888 as rgb, Framebuffer, VideoError, HEIGHT, WIDTH},
+};
+use gba_demos::raster_demo::{RasterDemo, RASTER_STATE, RASTER_TABLE};
+
+#[path = "scanlines/capture.rs"]
+mod capture;
+#[path = "scanlines/machine.rs"]
+mod machine;
+#[path = "scanlines/rendering.rs"]
+mod rendering;
+
+fn memory() -> Memory {
+    let mut m = Memory::new(vec![]).unwrap();
+    m.set_scanline_rendering(true);
+    m
+}
+
+fn present(m: &Memory) -> Framebuffer {
+    let mut f = Framebuffer::default();
+    assert!(m.present_frame(&mut f).unwrap());
+    f
+}
+
+fn at(f: &Framebuffer, x: usize, y: usize) -> u32 {
+    f.pixels()[y * WIDTH + x]
+}
+
+fn word_rom(code: &[u32]) -> Vec<u8> {
+    code.iter().flat_map(|v| v.to_le_bytes()).collect()
+}