Add postprocessing, add flakeModules.touchup

modules/flake.nix

@@ -1,4 +1,4 @@
-{ lib, ... }:
+{ config, lib, options, ... }:
 let
   inherit (lib)
     mkOption
@@ -33,6 +33,27 @@ in
         Raw flake output attributes. Any attribute can be set here, but some
         attributes are represented by options, to provide appropriate
         configuration merging.
+
+        Further processing may be applied to these attributes. See [`processedFlake`](flake-parts.md#opt-processedFlake) for more information.
+      '';
+    };
+    processedFlake = mkOption {
+      type = types.raw;
+      readOnly = true;
+      apply =
+        if options.processedFlake.isDefined
+        then x: x
+        else x: config.flake;
+      description = ''
+        The final flake output, as returned by `mkFlake`.
+
+        This separates two concerns:
+
+        - [`flake`](#opt-flake): the internal representation, where all attributes are available for use by other modules, regardless of whether they evaluate successfully for all configurations.
+        - `processedFlake`: the external output, which may have attributes removed or transformed to satisfy tools like `nix flake check`.
+
+        By default, `processedFlake` equals `flake` (no processing).
+        Import a module such as [`flakeModules.touchup`](#opt-touchup) to define it, or set it directly.
       '';
     };
   };