ENH: disable ensightWrite, vtkWrite function objects in post-process mode

- prevents accidental overwrite of runtime conversions.

src/functionObjects/utilities/vtkWrite/vtkWrite.C

@@ -56,6 +56,15 @@ Foam::functionObjects::vtkWrite::vtkWrite
     selectFields_(),
     dirName_("VTK")
 {
+    if (postProcess)
+    {
+        // Disable for post-process mode.
+        // Emit as FatalError for the try/catch in the caller.
+        FatalError
+            << type() << " disabled in post-process mode"
+            << exit(FatalError);
+    }
+
     read(dict);
 }