React Native adds Stable API guards to renderer attributed string

React Native's renderer attributed string module now carries a Stable API guard [1]. Under the three-tier C++ stable API visibility model, consumers that opt into `RN_STRICT_API` receive a warning if they include the module's headers directly; they can acknowledge the warning with `RN_ALLOW_FRAMEWORKS`. Without that flag the guards are inert, so existing behavior is unchanged unless strict mode is enabled.

As the commit describes, the target is classified as \"for frameworks,\" meaning it signals to framework consumers that the API is not intended for direct external use. This is a governance change rather than a runtime behavior change, aligning the module with the project's visibility road map.