Use dispatch_semaphore for eager-main-queue module setup gate

Summary:
Replace the `std::mutex` + `std::condition_variable` + `std::shared_ptr<bool>` trio that gates the JS thread on main-queue module construction with a single `dispatch_semaphore_t`. The wait block is invoked exactly once (from `_loadScriptFromSource:`'s `beforeLoad` lambda), so single-shot semaphore semantics fit the contract directly — no condition predicate, no shared state captured by three independent `shared_ptr`s.

Net: 12 lines deleted, 1 captured object instead of 3, no behavior change.

Changelog: [Internal]

Reviewed By: sammy-SC

Differential Revision: D105953979

fbshipit-source-id: 149e959d04e8466589798ae38c68059db77ecd8b

Author: javache

packages/react-native/ReactCommon/react/runtime/platform/ios/ReactCommon/RCTInstance.mm

@@ -379,13 +379,10 @@ - (void)_start
      */
     NSArray<NSString *> *modulesRequiringMainQueueSetup = [_delegate unstableModulesRequiringMainQueueSetup];
 
-    std::shared_ptr<std::mutex> mutex = std::make_shared<std::mutex>();
-    std::shared_ptr<std::condition_variable> cv = std::make_shared<std::condition_variable>();
-    std::shared_ptr<bool> isReady = std::make_shared<bool>(false);
+    dispatch_semaphore_t moduleSetupComplete = dispatch_semaphore_create(0);
 
     _waitUntilModuleSetupComplete = ^{
-      std::unique_lock<std::mutex> lock(*mutex);
-      cv->wait(lock, [isReady] { return *isReady; });
+      dispatch_semaphore_wait(moduleSetupComplete, DISPATCH_TIME_FOREVER);
     };
 
     // TODO(T218039767): Integrate perf logging into main queue module init
@@ -398,9 +395,7 @@ - (void)_start
       RCTScreenScale();
       RCTSwitchSize();
 
-      std::lock_guard<std::mutex> lock(*mutex);
-      *isReady = true;
-      cv->notify_all();
+      dispatch_semaphore_signal(moduleSetupComplete);
     });
   }