Thin Windows entry, preview shell, and platform legacy state

src/platform_legacy/legacy_platform_state.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#include "platform_api/platform_services.h"
+
+namespace pp::platform::legacy {
+
+[[nodiscard]] const pp::platform::PlatformStoragePaths& active_legacy_storage_paths();
+
+}