fix: asahi-nixos - revert mesa to 25.2.6

Signed-off-by: Ryan Yin <xiaoyin_c@qq.com>

hosts/12kingdoms-shoukei/apple-silicon.nix

@@ -2,6 +2,7 @@
   pkgs,
   nixos-apple-silicon,
   my-asahi-firmware,
+  nixpkgs-mesa,
   ...
 }:
 {
@@ -9,6 +10,13 @@
     nixos-apple-silicon.nixosModules.default
   ];
 
+  # Workaround for Mesa 25.3.0 regression
+  # https://github.com/nix-community/nixos-apple-silicon/issues/380
+  # https://github.com/NixOS/nixpkgs/pull/461866
+  hardware.graphics.package =
+    assert pkgs.mesa.version == "25.3.0";
+    (import nixpkgs-mesa { localSystem = pkgs.stdenv.hostPlatform; }).mesa;
+
   nix.settings = {
     extra-substituters = [
       "https://nixos-apple-silicon.cachix.org"