feat(ui): Select Other — pick what lies under something else (#503)

Hold the left button still (long-press on touch) or right-click › Select Other… to list everything under the cursor: sketch points and curves ("Line 3", "Point (arc 2 centre)"), vertices, edges, faces, bodies, profiles, planes and axes, filtered like a click and by the open dialog's active input. Hovering a row lights it up; clicking selects it (Shift/Ctrl add) or feeds the dialog. A repeat click on the same spot with nothing changed selects the next thing there; a double-click keeps its own actions.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/book/src/guide/ui-tour.md

@@ -14,7 +14,14 @@
   origin, bodies, sketches, construction planes, canvases, joints and components. The eye shows or
   hides an item, a component's radio button makes it the active component, and right-clicking any
   row gives that item's menu. Rows drag into groups and other components.
-- **Viewport.** Click to select, Shift or Ctrl to add, drag for a window or crossing box. The
+- **Viewport.** Click to select, Shift or Ctrl to add, drag for a window or crossing box. To
+  reach something under something else (two lines drawn on top of each other, coincident points,
+  an edge in front of a face, overlapping bodies), hold the left button still for half a second
+  (long-press on a touch screen) or right-click › **Select Other…**: a list of everything under
+  the cursor opens there, the sketch being edited first. Hovering a row lights it up; clicking it
+  selects it (Shift or Ctrl adds), and in a command's dialog it goes to the active selection box.
+  Esc or a click elsewhere closes the list. Clicking again on the same spot, with nothing changed
+  in between, selects the next thing in that list. The
   wheel zooms at the cursor, the middle button pans and right-drag orbits freely (the mouse
 <  preset in Preferences switches to SolidWorks, Inventor, Alias or Tinkercad style). On a
   trackpad, pinch zooms and two-finger scroll pans (Navigation › Two-finger scroll pans;