chore(dev): bootstrap Rust and Nix environments for Darwin

Configure direnv, native Darwin compiler flags, and pinned desktop dependencies. Exclude build outputs, local environments, ROMs, and saves.

Start an ordered import of existing work. Feature chunks share dependencies; validate the integrated stack, not each intermediate import.

Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "gba-rust"
+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"]