refactor(selection): split 31-field SelectionState into five sub-structs

SelectionState now holds view, gesture, input, menu and orbit sub-structs; all 450 access sites updated mechanically with no behavior change. Deletes three write-only dead fields (box_last, box_last_crossing, poly_last_crossing). Hand-rolled clears in commands and dim_viewport route through the new clear methods; bench fixture updated. Full suite: 1976 passed / 0 failed / 25 ignored.

src/scene/preview.rs

@@ -281,7 +281,7 @@ impl Scene {
         self.interim_wire = None;
         // Drop any point-picked window marquee (STRETCH) so it doesn't linger
         // after the command ends before the next mouse move. (#291)
-        self.selection.borrow_mut().preview_box = None;
+        self.selection.borrow_mut().gesture.preview_box = None;
     }
 
     pub fn wire_models_for(&self, handles: &[codec::Handle]) -> Vec<WireModel> {