* platform: popup window API with X11/Wayland support
- Add popup window type for context menus and dropdowns
- Wayland: xdg_popup with grab for compositor-driven dismiss
- X11: override-redirect windows with pointer grab
- Explicit-close semantics: app must handle PopupDismissed
- Fix Wayland crash on repeated context menu open/close
- Emit WindowClosed before PopupDismissed in PopupDone
* platform: native mobile selection handles
iOS: custom UIView selection handles with UIPanGestureRecognizer (all versions),
UITextSelectionDisplayInteraction for native highlights (iOS 16+),
MakepadSelectionRect for UITextInput protocol.
Android: custom SelectionHandleView with GradientDrawable oval, touch drag
listeners, JNI bridge for handle drag events.
Widgets: TextFlow clipboard action integration (show on touch up with selection,
hide on touch down/focus lost, TextCut handler), PortalList select-all and
clipboard actions, selection bounding rect computation for popup positioning.
Includes text_selection example app.
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Co-authored-by: ant <ant@offline.click>
