scroll_pane: screen_to_item() now properly accounts for wrapped
lines using textwrap to compute actual character positions instead
of just using mouse_x directly.
selectable: new module with PUA markers for wrap-aware selection.
Not yet integrated into chat.rs but ready for future use. Uses
continuation markers to track logical vs visual lines.
Co-Authored-By: Proof of Concept <poc@bcachefs.org>
