Cover react/renderer/observers/events:events with Stable API guards (#58121)

Summary:
Pull Request resolved: #58121

Classifies `react/renderer/observers/events:events` as a private target under the C++ stable API three-tier visibility model. Adds `#include <react/cxxstableapi/PrivateGuard.h>` to the module's only exported header (`EventPerformanceLogger.h`), and wires the guard dependency into BUCK and CMake. On iOS the module ships as the `React-Fabric/observers/events` subspec, and the parent `React-Fabric` spec already depends on `React-cxxstableapi` from an earlier migration, so no podspec change is needed.

`Scheduler.h` is a "for frameworks" header that still includes `EventPerformanceLogger.h`, so the module is not cleanly private until that include is replaced with a forward declaration. That change is in review separately (D116909093) and can land in either order relative to this one, since the guards are inert on their own.

The guards are inert unless a consumer defines `RN_STRICT_API`, so there is no behavior change.

Changelog: [Internal]

Reviewed By: christophpurrer

Differential Revision: D117320589

fbshipit-source-id: 4e17fb4d25b1a51b2c06391d77cbe071b01ac3ab

Author: j-piasecki

packages/react-native/ReactCommon/react/renderer/observers/events/CMakeLists.txt

@@ -13,6 +13,7 @@ add_library(react_renderer_observers_events OBJECT ${react_renderer_observers_ev
 
 target_include_directories(react_renderer_observers_events PUBLIC ${REACT_COMMON_DIR})
 target_link_libraries(react_renderer_observers_events
+        react_cxxstableapi
         react_debug
         react_performance_timeline
         react_timing

packages/react-native/ReactCommon/react/renderer/observers/events/EventPerformanceLogger.h

@@ -7,6 +7,8 @@
 
 #pragma once
 
+#include <react/cxxstableapi/PrivateGuard.h>
+
 #include <react/performance/timeline/PerformanceEntryReporter.h>
 #include <react/renderer/core/EventLogger.h>
 #include <react/renderer/runtimescheduler/RuntimeSchedulerEventTimingDelegate.h>