feat(flake): add fast build apps

Expose nix-fast-build through flake apps for checks and packages so repo validation can use parallel evaluation and build orchestration without replacing switch or activation workflows.

Author: khaneliman

flake/apps.nix

@@ -73,6 +73,20 @@ _: {
       groupApps = lib.mapAttrs' (
         name: value: lib.nameValuePair "update-${name}" (mkUpdateApp name value)
       ) inputGroups;
+
+      mkFastBuildApp = name: flakeRef: description: {
+        type = "app";
+        meta.description = description;
+        program = lib.getExe (
+          pkgs.writeShellApplication {
+            name = "fast-build-${name}";
+            runtimeInputs = [ pkgs.nix-fast-build ];
+            text = ''
+              nix-fast-build --flake ${flakeRef} --no-link "$@"
+            '';
+          }
+        );
+      };
     in
     {
       apps = groupApps // {
@@ -107,6 +121,13 @@ _: {
           program = lib.getExe (pkgs.callPackage ../packages/closure-analyzer/package.nix { });
         };
 
+        fast-build-checks =
+          mkFastBuildApp "checks" ".#checks"
+            "Evaluate and build checks with nix-fast-build";
+        fast-build-packages =
+          mkFastBuildApp "packages" ".#packages"
+            "Evaluate and build packages with nix-fast-build";
+
         update-plugins =
           let
             pythonWithRich = pkgs.python3.withPackages (ps: with ps; [ rich ]);