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/command_driver/mod.rs

@@ -1148,7 +1148,7 @@ impl OpenCADStudio {
         // The command picked the path; the hit test lives here, where
         // the camera and the drawing's geometry are. Everything after
         // matches what a lasso does, Remove included.
-        let canvas = self.tabs[i].scene.selection.borrow().vp_size;
+        let canvas = self.tabs[i].scene.selection.borrow().view.vp_size;
         let bounds = iced::Rectangle {
             x: 0.0,
             y: 0.0,