Keep TUI sidebar selections visible.

specs/tui.allium

@@ -53,6 +53,9 @@ surface TuiSurface {
         -- viewport width, keeps the cursor's visual row visible while
         -- scrolling, and highlights the cursor's source line. There is
         -- no line-number gutter.
+
+    @guarantee SidebarSelectionVisibility
+        -- The sidebar viewport follows the selected entry.
 }
 
 -- ─── Sidebar navigation ─────────────────────────────────────