selectable: silence unused method warning

The is_selected method is reserved for future per-character
highlight rendering when the module is fully integrated.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/user/selectable.rs

@@ -327,6 +327,7 @@ impl SelectableTextState {
     }
 
     /// Check if a logical position is within the current selection.
+    #[allow(dead_code)] // Reserved for future per-character highlight rendering
     fn is_selected(&self, logical_line: usize, col: usize) -> bool {
         let Some(ref sel) = self.selection else { return false };
         let (start, end) = sel.range();