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>

Cargo.toml

@@ -1,18 +1,8 @@
-[package]
-name = "gba-rust"
+[workspace]
+resolver = "2"
+members = ["crates/gba-core", "crates/gba-demos", "crates/desktop"]
+
+[workspace.package]
 version = "0.1.0"
 edition = "2021"
 publish = false
-
-[features]
-default = ["desktop"]
-desktop = ["dep:minifb"]
-
-[dependencies]
-# Pinned because src/desktop.rs corrects a macOS focus bug in this version.
-minifb = { version = "=0.28.0", optional = true, default-features = false, features = ["x11"] }
-
-[[bin]]
-name = "gba-rust"
-path = "src/main.rs"
-required-features = ["desktop"]