Add optional Nix development environment

.envrc

@@ -0,0 +1 @@
+use flake

README.md

@@ -412,3 +412,22 @@ See [doc/PERF-HANDOFF.md](doc/PERF-HANDOFF.md) and [doc/BENCHMARKS.md](doc/BENCH
 The historical Shen 22.4 head-to-head versus the `shen-c` 0.2.3 interpreter (same
 machine) is preserved in [doc/BENCHMARKS.md](doc/BENCHMARKS.md): fib 66–79× faster, n-queens ~2.5× faster,
 Einstein's riddle ~1.5× slower.
+## Optional Nix environment
+
+Nix is optional; the normal shen-lua build and launcher commands continue to work
+with tools installed by any method. For a pinned development toolchain:
+
+```sh
+nix develop
+```
+
+The flake also exports `packages.toolchain` for composition by
+[Bifrost](https://github.com/pyrex41/bifrost):
+
+```sh
+nix shell .#toolchain
+```
+
+If direnv is installed, `direnv allow` opts this checkout into the same dev
+shell automatically. Nothing activates until that explicit authorization, and
+Nix is never required at runtime.

flake.nix

@@ -0,0 +1,8 @@
+{
+  description = "shen-lua development environment";
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+  outputs = { nixpkgs, ... }: let systems = [ "aarch64-darwin" "x86_64-darwin" "aarch64-linux" "x86_64-linux" ]; each = f: nixpkgs.lib.genAttrs systems (system: f nixpkgs.legacyPackages.${system}); in {
+    packages = each (pkgs: { toolchain = pkgs.buildEnv { name = "shen-lua-toolchain"; paths = with pkgs; [ gnumake gcc luajit luarocks ]; }; default = pkgs.buildEnv { name = "shen-lua-toolchain"; paths = with pkgs; [ gnumake gcc luajit luarocks ]; }; });
+    devShells = each (pkgs: { default = pkgs.mkShell { packages = with pkgs; [ gnumake gcc luajit luarocks ]; }; });
+  };
+}