[Feature] Add role option to bottomTab for UITabBarSystemItem support

[IsaacIsrael]

Summary

Adds a new role property to OptionsBottomTab that maps to UITabBarItem(tabBarSystemItem:tag:) on iOS. This enables using system-provided tab bar items with their native icons and titles.

Supported roles: search, bookmarks, contacts, downloads, favorites, featured, history, more, mostRecent, mostViewed, recents, topRated.

iOS 26+ Liquid Glass

On iOS 26+, role: 'search' renders as a floating magnifying-glass button separated from the tab bar using Apple's new Liquid Glass design language — no extra configuration needed.

Usage

{
  component: {
    name: 'SearchScreen',
    options: {
      bottomTab: {
        role: 'search',
        // Optionally override the system icon:
        sfSymbol: 'magnifyingglass',
        iconColor: 'blue',
        selectedIconColor: 'red',
      }
    }
  }
}

Key behaviors

• Only one search per layout: If multiple tabs specify role: 'search', only the first is honored; subsequent ones fall back to a normal tab with an RCTLogWarn.
• Unknown roles fall back to a normal tab with a warning.
• Custom icons override system image: You can provide icon, selectedIcon, sfSymbol, or sfSelectedSymbol to replace the system-provided image after creation.
• iconColor and selectedIconColor are applied as tints to custom icons on role tabs.
• 'more' activates UIKit's built-in More navigation controller.
• Appearance chain preserved: standardAppearance and scrollEdgeAppearance are copied from the subclass chain onto system items.
• Instance-scoped tracking: Search role tracking uses an instance property on RNNTabBarItemCreator (not a static global), reset per layout.

Changes

• RNNBottomTabOptions.{h,mm} — parse role property
• RNNTabBarItemCreator.{h,mm} — searchRoleUsed instance property, createSystemItemForRole:mergeItem: with early returns, appearance chain copy, custom icon override with tinting
• BottomTabPresenter.{h,mm} — expose tabCreator as readonly property
• RNNBottomTabsController.mm — reset searchRoleUsed per layout
• src/interfaces/Options.ts — role union type with JSDoc
• Playground demo + Detox e2e test

Test plan

• Playground: "BottomTabs with Role" button shows search tab
• Detox: should render a search role tab e2e test
• Manual: Verify Liquid Glass floating button on iOS 26 simulator

[IsaacIsrael]

Updated PR — Addressing Review Feedback

Force-pushed a clean single commit rebased on latest master. Key changes from the previous version:

Removed

• tintColor on bottomTabs — removed as scope creep. Icon colors are handled via existing iconColor/selectedIconColor on custom images.
• tintColor commit — the second commit has been removed entirely.

Fixed

• Appearance chain: standardAppearance and scrollEdgeAppearance are now copied from the base item (created via subclass chain) onto system items, preserving tab bar styling.
• Static global → instance property: _rnn_searchRoleUsed replaced with searchRoleUsed instance property on RNNTabBarItemCreator, exposed via BottomTabPresenter.tabCreator. Reset per-layout in createTabBarItems:.
• Flattened nesting: Role handling extracted to createSystemItemForRole:mergeItem: with early returns for unknown roles and duplicate search tabs.

Improved

• JSDoc: Updated to document that custom icons override system images after creation, iconColor/selectedIconColor work as tints, 'search' triggers Liquid Glass on iOS 26+, and 'more' activates UIKit's More controller.

Added

• Playground demo: "BottomTabs with Role" button in LayoutsScreen showing a 3-tab layout with role: 'search'.
• Detox e2e test: Verifies the search role tab renders.