Replace rustPlatform.buildRustPackage with crane to split the Rust
compilation into a deps-only derivation (keyed on Cargo.lock) and a
source derivation. The deps derivation is cached in the Nix store
between CI runs, avoiding full recompilation on every deploy.
- Replace manual forAllSystems pattern with flake-parts.lib.mkFlake
- Use perSystem for packages and devShells definitions
- Pin rust-overlay's nixpkgs to follow main nixpkgs input
- No functional changes to package or dev shell
