Rust 1.85.0 + Rust 2024 Edition (#68)

- Add `rust-toolchain.toml`
- Refactor `flake` to consider `rust-toolchain.toml`, especially Rust version
- `just`: Add run-sound command
- Ignore sound files
- Format

.gitignore

@@ -18,3 +18,6 @@ result/**/*
 #.idea/
 #
 .direnv
+
+# ignore (possible) sound files
+**/*.{mp3,wav}