Cover react/renderer/dom with Stable API guards

Summary:
Classifies `react/renderer/dom` as a private target under the three-tier C++ stable API visibility model. `DOM.h` now includes `<react/cxxstableapi/PrivateGuard.h>`. Consumers that opt into `RN_STRICT_API` now get an error if they include `DOM.h` directly; without that flag the guard is inert, so no existing build changes behaviour.

Changelog: [Internal]

Differential Revision: D116013879

Author: j-piasecki

packages/react-native/ReactCommon/react/renderer/dom/DOM.h

@@ -7,6 +7,8 @@
 
 #pragma once
 
+#include <react/cxxstableapi/PrivateGuard.h>
+
 #include <react/renderer/components/root/RootShadowNode.h>
 #include <react/renderer/core/ShadowNode.h>
 #include <cstdint>