feat(edit): STRETCH crossing window selects objects implicitly (#338)

Bare STRETCH used to need a separate object-selection round before the
crossing window. Now the window itself picks the objects: with no prior
selection the command emits StretchWindow, the host gathers entities
whose world AABB touches the window (locked layers excluded) and
restarts at the base-point step. Over-selection is harmless since only
points inside the window move. The noun-verb flow (select first, then
STRETCH) is unchanged.

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

src/command.rs

@@ -908,6 +908,10 @@ pub enum CmdResult {
         scale: f32,
         angle: f32,
     },
+    /// Implicit STRETCH selection: the crossing window was drawn with no prior
+    /// selection, so the host resolves which entities it touches and restarts
+    /// the command at the base-point step with them. (#338)
+    StretchWindow { win_min: DVec3, win_max: DVec3 },
     /// Stretch entities: move only vertices/endpoints inside the crossing window.
     StretchEntities {
         handles: Vec<Handle>,