user: fix text selection on wrapped lines

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>

src/user/mod.rs

@@ -6,6 +6,7 @@
 pub(crate) mod chat;
 mod context;
 pub(crate) mod scroll_pane;
+pub mod selectable;
 mod subconscious;
 mod unconscious;
 mod thalamus;