fix(iOS): Fix memory leaks in RNSScreen for brownfield environments

[intmain]

Description

This PR fixes critical memory leaks in RNSScreen that occur when React Native is completely dismissed in brownfield applications. In brownfield environments, when RN views and the Hermes engine are fully torn down, RNSScreen and related objects were not being deallocated, causing significant memory leaks.

Root causes identified:

1. Retain cycle between RNSScreen ↔ RNSScreenView: UIViewController holds a strong reference to its view, and RNSScreenView holds a strong reference back to controller
2. CADisplayLink retain cycle: CADisplayLink holds a strong reference to its target (self)
3. Missing cleanup: No dealloc method and incomplete timer cleanup in setupProgressNotification
4. Lifecycle management: Controller needs temporary strong reference until parent takes ownership

Fixes memory leaks in brownfield React Native environments.

Changes

• Changed RNSScreenView.controller property from strong to weak reference
• Added _retainedController internal property to temporarily hold strong reference until parentViewController takes ownership
• Implemented RNSWeakProxy class using NSProxy pattern to break CADisplayLink retain cycle
• Added cleanup logic at the start of setupProgressNotification to handle multiple viewWillDisappear calls
• Added dealloc method to RNSScreen to properly clean up _animationTimer
• Added nil assignment after invalidate in completion block
• Release _retainedController in willMoveToParentViewController when parent takes ownership

Test code and steps to reproduce

Testing for memory leaks in brownfield environment:

Prerequisites: Brownfield iOS app with React Native integration

Steps:

1. Integrate this version of react-native-screens in your brownfield app
2. Present a React Native screen
3. Completely dismiss the React Native screen (tear down entire RN instance)
4. Use Xcode Instruments with "Leaks" template or check Allocations

Verification:
You can add a temporary log to verify dealloc is called:

// In RNSScreen.mm
- (void)dealloc
{
  NSLog(@"✅ RNSScreen dealloc called - memory leak fixed!");
  [_animationTimer invalidate];
  _animationTimer = nil;
}

When you completely dismiss RN, you should see the log message in console.

Expected behavior:

• Before fix: RNSScreen remains in memory, dealloc never called
• After fix: RNSScreen is properly deallocated, log appears in console

Testing in FabricExample:

cd FabricExample
yarn install
cd ios && pod install && cd ..
yarn ios
# Navigate through screens and observe memory behavior

Checklist

• Included code example that can be used to test this change ✅ (Testing instructions above)
• Updated TS types ❌ (Not applicable - iOS-only memory management fix)
• Updated documentation: ❌ (Not applicable - internal implementation fix, no API changes)
  • GUIDE_FOR_LIBRARY_AUTHORS.md
  • native-stack/README.md
  • src/types.tsx
  • src/native-stack/types.tsx
• Ensured that CI passes ⏳ (Will verify after PR submission)

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Note: This is an iOS-only internal implementation fix with no API changes, so TypeScript types and documentation updates are not required.