add stable nixpkgs to flake registry

flake.lock

@@ -355,6 +355,22 @@
         "type": "github"
       }
     },
+    "nixpkgs-stable": {
+      "locked": {
+        "lastModified": 1775811116,
+        "narHash": "sha256-t+HZK42pB6N+i5RGbuy7Xluez/VvWbembBdvzsc23Ss=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "54170c54449ea4d6725efd30d719c5e505f1c10e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-25.11",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "nixpkgs_2": {
       "locked": {
         "lastModified": 1774935083,
@@ -404,6 +420,7 @@
         "lanzaboote": "lanzaboote",
         "nixos-hardware": "nixos-hardware",
         "nixpkgs": "nixpkgs",
+        "nixpkgs-stable": "nixpkgs-stable",
         "simple-nixos-mailserver": "simple-nixos-mailserver",
         "vscode-extensions": "vscode-extensions"
       }