feat: Implement Thread-Safe Concurrency with Swift Actors and Kotlin Atomic Operations

[pushpender-singh-ap]

🎯 Overview

This PR introduces comprehensive thread-safety improvements using modern concurrency patterns - Swift Actors for iOS and Kotlin atomic operations with synchronized locks for Android.

🔄 What Changed

iOS Improvements

• Swift Actors: Introduced CameraSessionActor and CallbackActor for isolated state management
• Async/Await: Migrated to async patterns for non-blocking operations
• Race Condition Fixes: Eliminated check-then-act race conditions with atomic operations
• Memory Leak Prevention: Added proper cleanup in CameraView deinit
• TurboModule Migration: Removed RCTEventEmitter inheritance in favor of NativeReactNativeScannerSpecBase

Android Improvements

• Atomic Operations: Replaced @Synchronized with AtomicBoolean and AtomicReference
• ReentrantLock: Added fine-grained locking for camera binding operations
• Executor Management: Improved lifecycle with proper shutdown and recreation logic
• Thread-Safe Scanning: Lock-free scanning flag with CAS operations
• Choreographer Cleanup: Fixed memory leaks in ReactNativeScannerView

TypeScript/JavaScript

• CodegenTypes.EventEmitter: Migrated from manual event listeners to type-safe Codegen events
• Simplified API: Removed manual addListener/removeListeners methods
• Type Safety: Better event type definitions with BarcodeScannedEvent

📊 Technical Details

Thread Safety Patterns

Platform	Pattern	Purpose
iOS	Swift Actors	Isolated state management
iOS	Async/Await	Non-blocking operations
Android	AtomicBoolean/AtomicReference	Lock-free flag operations
Android	ReentrantLock	Camera binding synchronization
Android	Volatile	Visibility guarantees

🐛 Bugs Fixed

• ❌ Race condition in startScanning() between permission check and flag set
• ❌ Concurrent camera binding causing IllegalStateException
• ❌ Memory leaks from Choreographer callbacks
• ❌ Executor shutdown during active scanning
• ❌ Session interruption handling race conditions
• ❌ Preview layer binding timing issues

🔧 Migration Guide

Breaking Changes

1. Event Handling: No longer uses NativeEventEmitter - uses Codegen events automatically
2. iOS Session Access: currentSession() replaced with async getCurrentSession(completion:)

Before (v1.x):

import { NativeEventEmitter } from 'react-native';

const eventEmitter = new NativeEventEmitter(BarcodeScanner);
eventEmitter.addListener('onBarcodeScanned', callback);

After (v2.0.0):

// Handled automatically by the API
BarcodeScanner.startScanning(callback);

📈 Performance Impact

• Reduced Lock Contention: Atomic operations eliminate synchronized method overhead
• Non-Blocking iOS: Async patterns prevent main thread blocking
• Memory Efficiency: Proper cleanup prevents accumulation

📝 Documentation

• Updated README with thread-safety features
• Added inline code documentation for complex concurrency patterns