feat(m): package for nix

If we want to use menu on teal, it needs to be packaged for Nix. Let's
do that!

.gitignore

···
+# SPDX-FileCopyrightText: 2026 Freshly Baked Cake
+#
+# SPDX-License-Identifier: CC0-1.0
+
+result

menu/project.nix

···
 { config, lib }:
 {
   config = {
+    packages.default = config.packages.menu;
+    packages.menu = {
+      systems = [
+        "x86_64-linux"
+        "aarch64-linux"
+      ];
+
+      package =
+        {
+          fenix,
+          makeRustPlatform,
+          lib,
+          ...
+        }:
+        let
+          toolchain = fenix.complete.toolchain;
+
+          manifest = (lib.importTOML ./Cargo.toml).package;
+
+          platform = makeRustPlatform {
+            cargo = toolchain;
+            rustc = toolchain;
+          };
+        in
+        platform.buildRustPackage {
+          meta.mainProgram = "menu";
+          pname = manifest.name;
+          version = manifest.version;
+
+          src = ./.;
+
+          cargoLock.lockFile = ./Cargo.lock;
+        };
+    };
+
     shells.default = config.shells.menu;
     shells.menu = {
       systems = [ "x86_64-linux" ];