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/scene/view/render.rs

@@ -4213,7 +4213,7 @@ impl Scene {
         // `CursorMoved` handler — the cube overlay sits above the shader
         // and would otherwise mask the move event from `Program::update`.
         let hover_region = show_interaction.then(|| self.viewcube_hover.get()).flatten();
-        self.selection.borrow_mut().vp_size = (bounds.width, bounds.height);
+        self.selection.borrow_mut().view.vp_size = (bounds.width, bounds.height);
         if bounds.height > 0.0 {
             self.set_render_aspect(bounds.width / bounds.height);
             self.set_render_pixel_scale(bounds.width, bounds.height);