feat(measuregeom): Quick mode measures around the cursor

MEASUREGEOM gains a Quick option. As the cursor moves, the nearest
drawn geometry along the working plane's X and Y lines through it is
measured live: cyan measurement lines and a Horizontal/Vertical readout
by the cursor. A click prints the current readout and keeps measuring;
Enter ends the command. The rays go through the scene's interaction
index, so large drawings stay responsive.

A structure-pick command whose miss message is empty now gets the click
as a plain point instead of an error.

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

src/command.rs

@@ -2806,7 +2806,8 @@ pub trait CadCommand: Send {
         vec![]
     }
 
-    /// Message when `resolve_object_pick` returns none on click.
+    /// Message when `resolve_object_pick` returns none on click; an empty one
+    /// passes the click to `on_point` instead.
     fn object_pick_miss_message(&self) -> &'static str {
         "No object near click."
     }