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.lock

@@ -72,6 +72,27 @@
         "type": "github"
       }
     },
+    "nix-darwin": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1779036909,
+        "narHash": "sha256-zXcwYQGCT6pzinK+1dBB2ekTVtfxGZAapb3Evdcu4fY=",
+        "owner": "nix-darwin",
+        "repo": "nix-darwin",
+        "rev": "56c666e108467d87d13508936aade6d567f2a501",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-darwin",
+        "ref": "nix-darwin-26.05",
+        "repo": "nix-darwin",
+        "type": "github"
+      }
+    },
     "nixos-apple-silicon": {
       "inputs": {
         "flake-compat": "flake-compat",
@@ -149,6 +170,7 @@
       "inputs": {
         "flake-parts": "flake-parts",
         "home-manager": "home-manager",
+        "nix-darwin": "nix-darwin",
         "nixos-apple-silicon": "nixos-apple-silicon",
         "nixos-wsl": "nixos-wsl",
         "nixpkgs": "nixpkgs",