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>

src/effects_demo.rs

@@ -1,40 +0,0 @@
-//! Original CPU-driven window and color-effects scene, based on the tile demo.
-//! The CPU writes window bounds and effect registers during each VBlank.
-
-use crate::{
-    graphics_demo::{GraphicsDemo, GraphicsError},
-    input::Buttons,
-    machine::Machine,
-    memory::MemoryError,
-    video::Framebuffer,
-};
-
-/// Completed updates and nine-bit horizontal/vertical scroll, as three words.
-pub const EFFECTS_STATE: u32 = crate::tile_demo::TILE_STATE;
-
-pub struct EffectsDemo {
-    runner: GraphicsDemo,
-}
-
-impl EffectsDemo {
-    pub fn new() -> Result<Self, MemoryError> {
-        Ok(Self {
-            runner: GraphicsDemo::with_program(crate::tile_demo::effects_program())?,
-        })
-    }
-
-    pub fn machine(&self) -> &Machine {
-        self.runner.machine()
-    }
-
-    /// Arrows scroll and move WIN0. Z selects alpha; X selects darkening.
-    /// Otherwise the area outside WIN0 is brightened. Enter resets scrolling.
-    /// Q/W retain the tile demo's sprite rotation and zoom controls.
-    pub fn frame(
-        &mut self,
-        buttons: Buttons,
-        output: &mut Framebuffer,
-    ) -> Result<usize, GraphicsError> {
-        self.runner.frame_with_control(buttons, output, 0x3340)
-    }
-}