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/control/actions.rs

@@ -1496,7 +1496,7 @@ impl OpenCADStudio {
                     .filter(|v| v.is_finite())
                     .ok_or_else(|| failure("invalid_point", "Missing finite y"))?
                     as f32;
-                let size = self.tabs[self.active_tab].scene.selection.borrow().vp_size;
+                let size = self.tabs[self.active_tab].scene.selection.borrow().view.vp_size;
                 if x < 0. || y < 0. || x > size.0 || y > size.1 {
                     return Err(failure(
                         "outside_viewport",