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/app/dim_viewport.rs

@@ -20,10 +20,7 @@ pub(crate) enum DimensionMeasureSpace {
 impl OpenCADStudio {
     pub(crate) fn finish_command_click(&mut self, i: usize) {
         let mut selection = self.tabs[i].scene.selection.borrow_mut();
-        selection.left_down = false;
-        selection.left_press_pos = None;
-        selection.left_press_time = None;
-        selection.left_dragging = false;
+        selection.input.clear_left_buttons();
     }
 
     pub(crate) fn dimension_measure_space(&self, i: usize) -> DimensionMeasureSpace {