Route prepared export policy through platform services

src/platform_legacy/legacy_platform_services.cpp

@@ -431,6 +431,15 @@ public:
 #endif
     }
 
+    [[nodiscard]] bool uses_prepared_file_writes() override
+    {
+#if __IOS__ || __WEB__
+        return true;
+#else
+        return false;
+#endif
+    }
+
     [[nodiscard]] pp::platform::PreparedFileTarget prepare_writable_file(
         std::string_view type,
         std::string_view default_name,