feat: add selected to AdaptivePopupMenuItem (single-select checkmark)

[gem85247]

Summary

Adds a selected flag to AdaptivePopupMenuItem so a popup menu can act as a single-select control and show which option is currently active — the piece missing for using AdaptivePopupMenuButton as a settings-style value picker.

• iOS 14+ native menus: sets UIAction.state = .on, so the system renders its native trailing checkmark on the selected item.
• Material popup menus: shows a trailing check (Icons.check, colorScheme.primary).
• iOS <26 action-sheet fallback: shows a leading CupertinoIcons.checkmark.

The flag is plumbed through the iOS platform view (creationParams + updateMenuItems), added to _hasMenuItemsChanged and the view key, so toggling selection updates the live menu. Defaults to false, so this is fully backward compatible.

Why

Today there's no way to indicate the current value in a popup menu — you can only fake it by swapping an item's leading icon to a checkmark, which fights the native leading-image layout and never produces the real iOS trailing checkmark. isDestructive/subtitle/imageBytes already follow this same three-surface pattern; selected fills the gap.

Usage

AdaptivePopupMenuButton.text<String>(
  label: current,
  items: [
    for (final o in options)
      AdaptivePopupMenuItem(label: o, value: o, selected: o == current),
  ],
  onSelected: (_, item) => setState(() => current = item.value!),
)

Changes

• AdaptivePopupMenuItem.selected (default false) + dartdoc
• iOS 26 native: parse isSelected, apply UIAction.state
• Material + iOS <26 fallback: checkmark rendering
• README single-select example, CHANGELOG entry, version bump to 0.1.112

Test plan

• iOS 26 device/sim: selected item shows the native trailing checkmark; changing selection updates it live
• Android: selected item shows a trailing check in the Material menu
• iOS <26: selected item shows a leading checkmark in the action sheet
• Existing menus (no selected set) render unchanged

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