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.lock

@@ -141,12 +141,28 @@ dependencies = [
 ]
 
 [[package]]
-name = "gba-rust"
+name = "gameboy-rust-desktop"
 version = "0.1.0"
 dependencies = [
+ "gba-core",
+ "gba-demos",
  "minifb",
 ]
 
+[[package]]
+name = "gba-core"
+version = "0.1.0"
+dependencies = [
+ "gba-demos",
+]
+
+[[package]]
+name = "gba-demos"
+version = "0.1.0"
+dependencies = [
+ "gba-core",
+]
+
 [[package]]
 name = "instant"
 version = "0.1.13"