feat: add Nix flake for reproducible dev environment (#54)

flake.nix

@@ -0,0 +1,76 @@
+{
+  description = "LOOM - provably correct WebAssembly optimizer";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11";
+
+    rust-overlay = {
+      url = "github:oxalica/rust-overlay";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, rust-overlay, flake-utils }:
+    flake-utils.lib.eachSystem [
+      "x86_64-linux"
+      "aarch64-linux"
+      "x86_64-darwin"
+      "aarch64-darwin"
+    ] (system:
+      let
+        overlays = [ (import rust-overlay) ];
+        pkgs = import nixpkgs { inherit system overlays; };
+
+        # Rust stable with wasm32-wasip2 target
+        rustToolchain = pkgs.rust-bin.stable."1.93.1".default.override {
+          extensions = [
+            "rust-src"
+            "rust-analyzer"
+            "clippy"
+            "rustfmt"
+          ];
+          targets = [ "wasm32-wasip2" ];
+        };
+
+        # Z3 paths for the z3-sys crate build
+        z3Header = "${pkgs.z3.dev}/include/z3.h";
+        z3LibPath = "${pkgs.z3.lib}/lib";
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          buildInputs = [
+            rustToolchain
+
+            # Z3 SMT solver (for --features verification)
+            pkgs.z3
+
+            # WebAssembly tooling
+            pkgs.wasmtime
+            pkgs.binaryen
+
+            pkgs.pkg-config
+          ] ++ pkgs.lib.optionals pkgs.stdenv.isDarwin [
+            pkgs.darwin.apple_sdk.frameworks.Security
+            pkgs.darwin.apple_sdk.frameworks.SystemConfiguration
+            pkgs.libiconv
+          ];
+
+          env = {
+            Z3_SYS_Z3_HEADER = z3Header;
+            LIBRARY_PATH = z3LibPath;
+            LD_LIBRARY_PATH = z3LibPath;
+            RUST_SRC_PATH = "${rustToolchain}/lib/rustlib/src/rust/library";
+          };
+
+          shellHook = ''
+            echo "loom dev shell"
+            echo "  rust:     $(rustc --version)"
+            echo "  z3:       ${pkgs.z3.version}"
+            echo "  wasmtime: $(wasmtime --version 2>/dev/null)"
+          '';
+        };
+      }
+    );
+}