fix(iOS, Stack): center subview not visible/misaligned in header

[kligarski]

Description

Fixes missing/misaligned center subview in header on iOS.

before	after

Fixes #3482.

Changes

• limit wrapping subviews and related logic to left and right subviews only on iOS 26+ (previously, there was only an SDK macro, without runtime check)

Test code and steps to reproduce

Run Test3446 to verify that pressables are still working. To test center subview, use:

Test example

import React from 'react';
import { View, Text } from 'react-native';
import { createNativeStackNavigator } from '@react-navigation/native-stack';
import Colors from '../shared/styling/Colors';

function MainScreen() {
  return <View style={{ backgroundColor: 'moccasin' }} />;
}

const Stack = createNativeStackNavigator();

export default function App() {
  return (
    <Stack.Navigator>
      <Stack.Screen
        name="Main"
        component={MainScreen}
        options={{
          headerLeft: () => <Text>a</Text>,
          headerTitle: () => <Text>b</Text>,
          headerRight: () => <Text>c</Text>,
          headerBackground: () => (
            <View
              style={{
                backgroundColor: Colors.PurpleLight80,
                width: '100%',
                height: 120,
              }}
            />
          ),
        }}
      />
    </Stack.Navigator>
  );
}

Checklist

• Included code example that can be used to test this change
• Ensured that CI passes

[kkafar]

Looks good. I haven't checked the runtime, so please test it carefully & land it when you are positive it works.

[kligarski]

I tested both Fabric and Paper using test provided in PR description, Test3446 and TestHeaderTitle. Did not find any regressions.

[joaquinvaz]

@kkafar Hi! please we need this on a new release to upgrade our app, thanks!