feat(compare): COMPARE overlays what differs from another drawing

COMPARE picks a drawing (or takes its path) and matches its objects with
this drawing's by what they draw: type, layer and tessellated geometry,
so an unchanged copy matches whatever its handle. Objects only here are
outlined in green, objects only in the other drawing in red, over the
current drawing, and Next / Previous zoom from one difference to the
next with a count in the prompt. A moved or edited object shows as one
of each. eXit (or Esc) clears the overlay.

Refs #970

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/command.rs

@@ -1948,6 +1948,11 @@ pub enum CmdResult {
         p1: DVec3,
         p2: DVec3,
     },
+    /// Zoom to the given corner points; the command stays active.
+    Show {
+        p1: DVec3,
+        p2: DVec3,
+    },
     /// Commit `entity`, then zoom to the given corner points; end command.
     CommitAndZoom {
         entity: EntityType,