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>

tests/scanlines.rs

@@ -1,38 +0,0 @@
-use gba_rust::{
-    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},
-    raster_demo::{RasterDemo, RASTER_STATE, RASTER_TABLE},
-    video::{rgb555_to_rgb888 as rgb, Framebuffer, VideoError, HEIGHT, WIDTH},
-};
-
-#[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()
-}