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/mtext_editor.rs

@@ -721,7 +721,7 @@ impl super::OpenCADStudio {
             return;
         }
         let mut state = MTextEditorState::new(pos, initial, height, handle);
-        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;
         }
         // Seed attachment / line-spacing / box width from the entity being edited.