fix(ios): add thread-safety to HybridVideoPlayerEventEmitter listeners

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Summary

HybridVideoPlayerEventEmitter.listeners is accessed from multiple threads without synchronization:

• Event emission (emitEvent) happens from Swift concurrency async contexts — e.g. SourceLoaderActor.load() → HybridVideoPlayer.initializePlayerItem() → onLoadStart() runs on a cooperative thread pool queue.
• Listener add/remove happens from the JS thread via addListener / subscription remove closures.

This causes EXC_BAD_ACCESS (SIGSEGV) crashes in _swift_release_dealloc when the array is mutated during iteration, as Swift's copy-on-write optimization leaves a dangling buffer pointer.

Crash stack (abbreviated)

Thread 4 (crashed):
  libswiftCore.dylib  _swift_release_dealloc
  ReactNativeVideo    HybridVideoPlayerEventEmitter.emitEvent<τ_0_0>(…)
  ReactNativeVideo    HybridVideoPlayerEventEmitter.onLoadStart(…)
  ReactNativeVideo    HybridVideoPlayer.initializePlayerItem(…)
  ReactNativeVideo    closure #1 in HybridVideoPlayer.loadSource(…)

Fix

• Protect all reads/writes to the listeners array with an NSLock
• Snapshot the array before iterating in emitEvent so callbacks execute outside the lock (avoids deadlocks, keeps the critical section minimal)
• Make the backing store private to prevent unprotected external access

Test plan

• Reproduced the crash in a production app with rapid video source changes
• Applied the fix via patch-package — crash no longer reproduces
• Verified event delivery still works correctly (onLoadStart, onLoad, onProgress, onEnd all fire)