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/src/input.rs

@@ -0,0 +1,42 @@
+//! Platform-independent GBA button state. Bits are pressed-high here;
+//! the KEYINPUT register exposes their active-low hardware representation.
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(u16)]
+pub enum Button {
+    A = 1 << 0,
+    B = 1 << 1,
+    Select = 1 << 2,
+    Start = 1 << 3,
+    Right = 1 << 4,
+    Left = 1 << 5,
+    Up = 1 << 6,
+    Down = 1 << 7,
+    R = 1 << 8,
+    L = 1 << 9,
+}
+
+#[derive(Debug, Default, Clone, Copy, PartialEq, Eq)]
+pub struct Buttons(u16);
+
+impl Buttons {
+    pub fn from_bits(bits: u16) -> Self {
+        Self(bits & 0x03ff)
+    }
+
+    pub fn bits(self) -> u16 {
+        self.0
+    }
+
+    pub fn with(self, button: Button, pressed: bool) -> Self {
+        Self(if pressed {
+            self.0 | button as u16
+        } else {
+            self.0 & !(button as u16)
+        })
+    }
+
+    pub(crate) fn keyinput(self) -> u16 {
+        !self.0 & 0x03ff
+    }
+}