Expose UINavigationItem.style in Stack v5 iOS header options

[juliusmarminge]

I would like to expose UIKit's UINavigationItem.style on Stack v5 headers, following the existing platform options shape:

<Stack.HeaderConfig
  title="Document"
  ios={{ navigationItemStyle: 'editor' }}
/>

The proposed type is StackHeaderNavigationItemStyleIOS = 'navigator' | 'browser' | 'editor', exported with the other Stack header types. navigationItemStyle belongs to StackHeaderConfigPropsIOS, since it is a navigation-item layout policy rather than an appearance attribute or a shared cross-platform option.

These are the three UIKit cases, available from iOS 16:

• navigator uses the traditional navigation layout and is UIKit's default.
• browser aligns the title to the leading side and supports center item groups.
• editor uses the leading title layout and a chevron-only back button.

References: UINavigationItem.ItemStyle, style.

The option would map directly to the native enum. Omitting the option, removing it during an update, or removing the header config must restore navigator. On systems before iOS 16 and on tvOS it would be ignored; Android would drop the ios option through its existing wrapper. It would not add center items, toolbar placement, search behavior, or back-button customization.

A minimal FabricExample scenario would switch between all three cases on a pushed screen, reset by removing the option, remove/remount the header config, and navigate between screens with different styles. Native verification would use matching base/head captures plus live updates on iPhone and iPad, since UIKit controls the layout by idiom and available space.

This proposal generalizes a boolean editor-style implementation from T3 Code's local v5 patch into the full UIKit enum. It is independent of the missing native implementation of the existing backButtonHidden prop. The legacy back-button title/menu work in #3303 does not expose navigation-item style.

For React Navigation, main does not yet have a Stack v5 adapter. I plan a companion patch against the v5 adoption draft #13115, and will keep that integration gate explicit on the Screens PR.

Does ios.navigationItemStyle with the complete UIKit enum fit the direction for Stack v5 headers?