Convert: turn associative dimensions into dimensional constraints

The command deleted the dimension and left a literal constraint behind. The
reference keeps the dimension, drives it with a new parameter and gathers
only associative dimensions, saying how many objects it dropped.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/command.rs

@@ -2782,6 +2782,13 @@ pub trait CadCommand: Send {
         false
     }
 
+    /// The command takes associative dimensions only. The host drops every
+    /// other object from a completed selection, says how many it dropped, and
+    /// hands the command what is left.
+    fn selection_keeps_associative_dimensions(&self) -> bool {
+        false
+    }
+
     /// Called after a selection action completes while `is_selection_gathering` is true.
     /// `handles` is the full set of currently selected entities.
     /// Return `Relaunch` to fire the pending command, or `NeedPoint` to keep gathering.