fix(iOS, Stack v4): prevent header subview memory leak (#3653)

## Description

Prevents header subview memory leak.

Closes
software-mansion/react-native-screens-labs#696.

### Details

When header items were introduced, potential memory leak was spotted:
#2987 (comment).

From my testing, it seems that after introducing a wrapper view on iOS
26+
(#3449),
UIKit manages to deallocate all necessary views. On iOS 18 however I
managed to create a memory leak (see video "before").

There is one thing that confuses me a little bit - it seems like on iOS
26, UIKit retains the reference to bar button item & wrapper view a
little bit longer than expected (e.g. via
`_UINavigationBarContentView`). When we push a screen with bar button
item, pop it, it seems like the reference remains but when we push a new
screen, it is deallocated.

Also, when first screen uses `headerShown: false`, the behavior seems
similar also on iOS 18 but this time with header subview reference. It
is also deallocated after pushing another screen.

I don't think that we need to worry about this for now but I wanted to
point that out in PR description.

### Solution

I decided to keep the reference to `UIBarButtonItem` inside
`RNSScreenStackHeaderSubview` to maintain prop update logic
(#2987 (comment)).
I don't think that we can make it a weak reference because in header
config we clear left/rightBarButtonItems from navigation item - I guess
that the button might be deallocated before we attempt to re-use it
(correct me if I'm wrong).

Because of the above, I needed a place to clear the reference. I thought
about willMoveToWindow/Superview callbacks but:
1. `willMoveToWindow` is called multiple times so it's not reliable
2. `willMoveToSuperview` is better but when we push another screen over
a screen with bar button item, we will move to nil. I think that we
don't want to clear the bar button then to maintain any (internal) state
inside the button.

At first, I wanted to use already existing logic in header config's
`unmountChildComponentView` but this callback's Paper counterpart isn't
called when entire screen is popped instead of only subview. That's why
I used `invalidate` callback. On Paper, it's available via
`RCTInvalidating`. On Fabric, this is available on RCTViewComponentView
starting from RN 0.82. As next release will require RN 0.82 for Fabric,
we can comfortably use this callback.

## Changes

- add method to clear bar button reference
- use it in `invalidate` callback

## Before & after - visual documentation

### Before


https://github.com/user-attachments/assets/fb07fd55-8462-4fec-9791-85733c78e606

### After


https://github.com/user-attachments/assets/42ab5c6c-4a97-4db1-a098-60d08d39adc8

## Test plan

Run `Test3422`. Remove `headerShown: false` from `Screen1`. Push a
screen and then pop it. Check memory graph for `HeaderSubview`. You can
also override `dealloc` for header subview.

## Checklist

- [x] Included code example that can be used to test this change.
- [ ] Ensured that CI passes

Author: kligarski

ios/RNSScreenStackHeaderSubview.h

@@ -14,7 +14,7 @@ NS_ASSUME_NONNULL_BEGIN
 #ifdef RCT_NEW_ARCH_ENABLED
     RCTViewComponentView
 #else
-    UIView
+    UIView <RCTInvalidating>
 #endif
 
 @property (nonatomic) RNSScreenStackHeaderSubviewType type;
@@ -49,6 +49,9 @@ NS_ASSUME_NONNULL_BEGIN
 - (void)updateShadowStateInContextOfAncestorView:(nullable UIView *)ancestorView withFrame:(CGRect)frame;
 #endif
 
+/**
+ * Returns UIBarButtonItem associated with this subview. If it doesn't exist, UIBarButtonItem is created.
+ */
 - (UIBarButtonItem *)getUIBarButtonItem;
 
 @end

ios/RNSScreenStackHeaderSubview.mm

@@ -27,8 +27,8 @@ @implementation RNSScreenStackHeaderSubview {
 #if !RCT_NEW_ARCH_ENABLED && RNS_IPHONE_OS_VERSION_AVAILABLE(26_0)
   CGSize _lastReactFrameSize;
 #endif // !RCT_NEW_ARCH_ENABLED && RNS_IPHONE_OS_VERSION_AVAILABLE(26_0)
-  // TODO: Refactor this, so that we don't keep reference here at all.
-  // Currently this likely creates retain cycle between subview & the bar button item.
+  // This is a strong reference to UIBarButtonItem which creates a retain cycle.
+  // The cycle is cleared via `invalidateUIBarButtonItem` method, called by `invalidate` callback.
   UIBarButtonItem *_barButtonItem;
   BOOL _hidesSharedBackground;
 }
@@ -223,6 +223,12 @@ - (void)reactSetFrame:(CGRect)frame
 
 #endif // RCT_NEW_ARCH_ENABLED
 
+// Used by both Fabric & Paper
+- (void)invalidate
+{
+  [self invalidateUIBarButtonItem];
+}
+
 #if RNS_IPHONE_OS_VERSION_AVAILABLE(26_0)
 
 // Starting from iOS 26, to center left and right subviews inside liquid glass backdrop,
@@ -278,7 +284,7 @@ - (UIBarButtonItem *)getUIBarButtonItem
       // 2. Set content hugging priority for RNSScreenStackHeaderSubview.
       [self setContentHuggingPriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisHorizontal];
       [self setContentHuggingPriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisVertical];
-      
+
       // 3. Set compression resistance to prevent UIKit from shrinking the subview below its intrinsic size.
       [self setContentCompressionResistancePriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisVertical];
       [self setContentCompressionResistancePriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisHorizontal];
@@ -295,6 +301,15 @@ - (UIBarButtonItem *)getUIBarButtonItem
   return _barButtonItem;
 }
 
+- (void)invalidateUIBarButtonItem
+{
+  if (_type != RNSScreenStackHeaderSubviewTypeLeft && _type != RNSScreenStackHeaderSubviewTypeRight) {
+    return;
+  }
+
+  _barButtonItem = nil;
+}
+
 #if RNS_IPHONE_OS_VERSION_AVAILABLE(26_0)
 - (CGSize)intrinsicContentSize
 {