chore: add flake.nix

flake.nix

@@ -0,0 +1,87 @@
+{
+  description = "ROS 2 Rust client library (rclrs)";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    rust-overlay = {
+      url = "github:oxalica/rust-overlay";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs = { self, nixpkgs, rust-overlay, ... }:
+    let
+      supportedSystems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
+      forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
+
+      pkgsFor = system: import nixpkgs {
+        inherit system;
+        overlays = [ rust-overlay.overlays.default ];
+      };
+    in
+    {
+      devShells = forAllSystems (system:
+        let
+          pkgs = pkgsFor system;
+
+          rustToolchain = pkgs.rust-bin.stable.latest.default.override {
+            extensions = [ "rust-src" "rust-analyzer" "clippy" ];
+          };
+        in
+        {
+          default = let
+            # Wrapper script that adds --features use_ros_shim to cargo commands
+            cargoWrapper = pkgs.writeShellScriptBin "cargo" ''
+              args=("$@")
+              cmd="''${args[0]:-}"
+
+              case "$cmd" in
+                build|test|doc|check|clippy|run)
+                  exec ${rustToolchain}/bin/cargo "$@" --features use_ros_shim
+                  ;;
+                *)
+                  exec ${rustToolchain}/bin/cargo "$@"
+                  ;;
+              esac
+            '';
+          in pkgs.mkShell {
+            name = "ros2-rust";
+
+            nativeBuildInputs = [
+              cargoWrapper
+              rustToolchain
+              pkgs.pkg-config
+              pkgs.llvmPackages.libclang
+            ];
+
+            LIBCLANG_PATH = "${pkgs.llvmPackages.libclang.lib}/lib";
+            RUSTFLAGS = "--cfg ros_distro=\"humble\"";
+
+            shellHook = ''
+              echo "═══════════════════════════════════════════════════════"
+              echo "  ROS 2 Rust Development Environment"
+              echo "═══════════════════════════════════════════════════════"
+              echo "  Rust:   $(${rustToolchain}/bin/rustc --version)"
+              echo ""
+              echo "  cargo build/test/doc automatically use --features use_ros_shim"
+              echo "═══════════════════════════════════════════════════════"
+            '';
+          };
+        });
+
+      checks = forAllSystems (system:
+        let
+          pkgs = pkgsFor system;
+        in
+        {
+          fmt = pkgs.runCommand "check-fmt" {
+            nativeBuildInputs = [ pkgs.rust-bin.stable.latest.default ];
+            src = self;
+          } ''
+            cd $src
+            cargo fmt -- --check
+            touch $out
+          '';
+        });
+    };
+}