* selection: clipboard delegate, primary selection, mobile UI, handles, accessibility
Level 0: Route clipboard through Makepad instead of arboard. Custom
ClipboardDelegate in havishell forwards set_text/get_text/clear through
Makepad's CopyToClipboard and pending paste state. Fix Wayland serial
constraint by queuing CopyToClipboard when no serial is available.
Level 1: Primary selection (Linux). Add CxOsOp::SetPrimarySelection,
Wayland zwp_primary_selection_device_manager_v1 protocol bindings, X11
PRIMARY atom handling. HAVI stores selection text in SharedDocumentSelection
and calls set_primary_selection on change.
Level 2: Mobile clipboard actions UI. Long-press selects word and shows
native clipboard toolbar. TextCopy/TextCut events return selection text.
Level 3: Selection handle API. Add CxOsOp Show/Update/HideSelectionHandles,
Event::SelectionHandleDrag, and HAVI integration for handle drag events.
Platform stubs for all backends.
Level 4: Accessibility plumbing. Add CxOsOp::AccessibilityUpdate with
type-erased Box<dyn Any + Send> payload. HAVI implements
notify_accessibility_tree_update to forward accesskit::TreeUpdate through
Makepad. Platform no-op stubs.
* wayland: handle primary selection data_offer child objects
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Co-authored-by: ant <ant@offline.click>
