fix(ios): use positional index for SwiftUI TabView tags

[ios-youngsun-ban]

Summary

Fixes #1079.

In the new SwiftUI TabView-based iOS implementation (ios/PagerView.swift),
the tag for each page is derived with props.children.firstIndex(of:) while
iterating that same array inside ForEach:

ForEach(props.children) { child in
  if let index = props.children.firstIndex(of: child) {
    RepresentableView(view: child.view).tag(index)
  }
}

firstIndex(of:) returns the index of the first Equatable-equal element,
so equal entries resolve to the same index and produce duplicate .tag
values. With TabView(selection:), duplicate tags break page selection — and
on iOS 17 and below this surfaces as a hard crash
(NSInternalInconsistencyException). The lookup is also O(n²) over the
ForEach.

This PR derives the tag from the positional index via enumerated() (unique by
construction) while keeping stable SwiftUI identity through the element id:

ForEach(Array(props.children.enumerated()), id: \.element.id) { index, child in
  RepresentableView(view: child.view).tag(index)
}

Impact: iOS new-architecture (Fabric) implementation only. Identity diffing is
unchanged (still keyed by IdentifiablePlatformView.id); the change only makes
the tag unique by construction, removing the crash and the O(n²) lookup.

Test Plan

Verified on a device and a simulator running iOS 17 (and below): before this
change, paging throws NSInternalInconsistencyException and crashes; after
applying the patch the crash is gone and paging works as expected. Behavior is
otherwise unchanged — the positional index matches the previous firstIndex
result in normal states.

What's required for testing (prerequisites)?

• iOS example app on the new architecture (Fabric), running on iOS 17 or below.

What are the steps to reproduce (after prerequisites)?

• Render <PagerView> with multiple children and swipe / change pages.
  Before the fix this crashes with NSInternalInconsistencyException on iOS ≤ 17;
  after the fix paging works normally.

Compatibility

OS	Implemented
iOS	✅
Android	❌

Checklist

• I have tested this on a device and a simulator
• I added the documentation in README.md
• I updated the typed files (TS and Flow)

[Copilot]

Pull request overview

Fixes an iOS SwiftUI TabView(selection:) crash/wrong-selection scenario in the new-architecture PagerView implementation by ensuring each page’s .tag is unique and derived from position (not Equatable equality).

Changes:

• Replace firstIndex(of:) (O(n²) and can yield duplicate indices) with positional indices via enumerated().
• Preserve SwiftUI identity diffing by keeping the ForEach identity keyed on child.id (IdentifiablePlatformView.id).

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