feat(darwin): add nix-darwin support with a default macOS host

Add the nix-darwin input (nix-darwin-26.05, follows nixpkgs) and a mkDarwinHost mirroring mkHost: shared commonModule (nixpkgs/nix settings) is factored out and reused, home-manager is wired via darwinModules, and identity is threaded through specialArgs.

New darwinConfigurations.lyrathorpe-mac (aarch64-darwin) reuses the cross-platform ./lyrathorpe/home modules (shell, git, editor); Linux-only sway/desktop modules are excluded. Build with: darwin-rebuild switch --flake .#lyrathorpe-mac.

flake.nix

@@ -15,6 +15,9 @@
     # Apple Silicon (Asahi) support for the MacBook host.
     nixos-apple-silicon.url = "github:nix-community/nixos-apple-silicon";
     nixos-apple-silicon.inputs.nixpkgs.follows = "nixpkgs";
+    # nix-darwin: manage macOS hosts from this same flake.
+    nix-darwin.url = "github:nix-darwin/nix-darwin/nix-darwin-26.05";
+    nix-darwin.inputs.nixpkgs.follows = "nixpkgs";
     # Provides mkFlake: the systems/perSystem scaffolding used below.
     flake-parts.url = "github:hercules-ci/flake-parts";
     flake-parts.inputs.nixpkgs-lib.follows = "nixpkgs";
@@ -28,6 +31,7 @@
       home-manager,
       nixos-wsl,
       nixos-apple-silicon,
+      nix-darwin,
       ...
     }:
     flake-parts.lib.mkFlake { inherit inputs; } (
@@ -51,20 +55,23 @@
           "lens-desktop"
         ];
 
-        # Shared scaffolding for every host: common user, overlays, home-manager.
+        # nixpkgs + nix-daemon settings shared by NixOS and Darwin hosts.
+        commonModule = {
+          nixpkgs.overlays = overlays;
+          nixpkgs.config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) unfreePackages;
+          nix.settings.experimental-features = [
+            "nix-command"
+            "flakes"
+          ];
+          # Make `nix shell nixpkgs#...` and <nixpkgs> use the pinned nixpkgs.
+          nix.registry.nixpkgs.flake = nixpkgs;
+          nix.nixPath = [ "nixpkgs=${nixpkgs}" ];
+        };
+
+        # Shared scaffolding for every NixOS host: common user, settings, home-manager.
         baseModules = [
           ./lyrathorpe/user.nix
-          {
-            nixpkgs.overlays = overlays;
-            nixpkgs.config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) unfreePackages;
-            nix.settings.experimental-features = [
-              "nix-command"
-              "flakes"
-            ];
-            # Make `nix shell nixpkgs#...` and <nixpkgs> use the pinned nixpkgs.
-            nix.registry.nixpkgs.flake = nixpkgs;
-            nix.nixPath = [ "nixpkgs=${nixpkgs}" ];
-          }
+          commonModule
           home-manager.nixosModules.home-manager
           {
             home-manager.useGlobalPkgs = true;
@@ -99,6 +106,43 @@
               ];
           };
 
+        # Shared scaffolding for every Darwin (macOS) host.
+        darwinBaseModules = [
+          commonModule
+          home-manager.darwinModules.home-manager
+          {
+            home-manager.useGlobalPkgs = true;
+            home-manager.useUserPackages = true;
+          }
+        ];
+
+        # mkDarwinHost :: { system, username, fullName, modules, homeModules } -> darwinSystem
+        # Darwin counterpart of mkHost. macOS already owns the login user, so we
+        # only attach the platform and home-manager; no NixOS user module here.
+        mkDarwinHost =
+          {
+            system,
+            username,
+            fullName,
+            modules,
+            homeModules,
+          }:
+          nix-darwin.lib.darwinSystem {
+            specialArgs = { inherit inputs username fullName; };
+            modules =
+              darwinBaseModules
+              ++ modules
+              ++ [
+                {
+                  nixpkgs.hostPlatform = system;
+                  # macOS owns the account; point home-manager at its home dir.
+                  users.users.${username}.home = "/Users/${username}";
+                  home-manager.extraSpecialArgs = { inherit inputs username fullName; };
+                  home-manager.users.${username}.imports = homeModules;
+                }
+              ];
+          };
+
         # Host table — declarative registry of every machine. To add a host:
         # give it a name, its `system`, the owning user, and the module lists.
         # mapAttrs below turns each entry into a nixosConfiguration of the same name.
@@ -147,11 +191,30 @@
             ];
           };
         };
+
+        # Darwin host table — macOS machines built via mkDarwinHost. The shared
+        # ./lyrathorpe/home modules (shell, git, editor) are reused; the Linux-only
+        # desktop/sway modules are intentionally left out.
+        darwinHosts = {
+          lyrathorpe-mac = {
+            system = "aarch64-darwin";
+            username = "lyrathorpe";
+            fullName = "Lyra Thorpe";
+            modules = [
+              ./system/machine/Darwin/configuration.nix
+            ];
+            homeModules = [
+              ./lyrathorpe/home
+            ];
+          };
+        };
       in
       {
         systems = [
           "x86_64-linux"
           "aarch64-linux"
+          "aarch64-darwin"
+          "x86_64-darwin"
         ];
 
         # perSystem is evaluated once per entry in `systems`; `pkgs` is the
@@ -180,8 +243,9 @@
                 '';
           };
 
-        # Realise the host table: each `hosts` entry becomes a nixosConfiguration.
+        # Realise the host tables: each entry becomes a {nixos,darwin}Configuration.
         flake.nixosConfigurations = lib.mapAttrs (_name: mkHost) hosts;
+        flake.darwinConfigurations = lib.mapAttrs (_name: mkDarwinHost) darwinHosts;
       }
     );
 }