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/text_inline.rs

@@ -214,7 +214,7 @@ impl super::OpenCADStudio {
             creation,
             screen_anchor: iced::Point::new(60.0, 90.0),
         };
-        if let Some(p) = self.tabs[self.active_tab].scene.selection.borrow().last_move_pos {
+        if let Some(p) = self.tabs[self.active_tab].scene.selection.borrow().input.last_move_pos {
             state.screen_anchor = p;
         }
         self.text_inline = Some(state);