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/effects_demo.rs

@@ -0,0 +1,35 @@
+//! 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};
+use gba_core::{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)
+    }
+}