Add Nix flake and direnv

Dev shell for x86_64-linux, aarch64-linux and aarch64-darwin. The toolchain
comes from rust-toolchain.toml with rust-src and rust-analyzer, and .envrc
loads the make-style .env.

.envrc

@@ -0,0 +1,8 @@
+# .env is make syntax (unquoted values with spaces), so direnv's dotenv rejects it.
+if [[ -f .env ]]; then
+  watch_file .env
+  while IFS='=' read -r key value; do
+    [[ $key =~ ^[A-Za-z_][A-Za-z0-9_]*$ ]] && export "$key=$value"
+  done < .env
+fi
+use flake

.gitignore

@@ -14,3 +14,8 @@ iw4l.community.*.tmp
 # Agent working memory. None of it is the product: the artifacts, the externals,
 # the agent clones and the RE tooling and its bases all stay on this machine.
 /context/
+
+# Nix / direnv
+.direnv/
+result
+result-*

flake.lock

@@ -0,0 +1,82 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1791260994,
+        "narHash": "sha256-Miqqk/ammqnTUxaoCyvtwoPeLbI1ksFyLxi/CazZpWY=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "151fa4e8ddfdd8dd25d945ad94ed54a13de9f6e4",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs",
+        "rust-overlay": "rust-overlay"
+      }
+    },
+    "rust-overlay": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1791361643,
+        "narHash": "sha256-HMdfSb1ONHs1RbSkQtCXm+tWEsTikisWWtXDnv0aQ+I=",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "8afee9fa8caa877a4feb65adc33e8d72f3747dc9",
+        "type": "github"
+      },
+      "original": {
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "type": "github"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,64 @@
+{
+  description = "iw4L development shell";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    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" "aarch64-darwin" ] (
+      system:
+      let
+        pkgs = import nixpkgs {
+          inherit system;
+          overlays = [ (import rust-overlay) ];
+        };
+        toolchain = (pkgs.rust-bin.fromRustupToolchainFile ./rust-toolchain.toml).override {
+          extensions = [
+            "rust-src"
+            "rust-analyzer"
+          ];
+        };
+        runtimeLibs = pkgs.lib.optionals pkgs.stdenv.hostPlatform.isLinux (
+          with pkgs;
+          [
+            alsa-lib
+            libxkbcommon
+            udev
+            vulkan-loader
+            wayland
+            libx11
+            libxcursor
+            libxi
+            libxrandr
+          ]
+        );
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          nativeBuildInputs = with pkgs; [
+            toolchain
+            pkg-config
+            cmake
+            clang
+            git
+          ];
+          buildInputs = runtimeLibs;
+          LD_LIBRARY_PATH = pkgs.lib.optionalString pkgs.stdenv.hostPlatform.isLinux (
+            pkgs.lib.makeLibraryPath runtimeLibs
+          );
+        };
+      }
+    );
+}