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/properties.rs

@@ -2065,7 +2065,7 @@ impl OpenCADStudio {
                     let doc = &scene.document;
                     let header = &doc.header;
                     let camera = scene.camera.borrow();
-                    let (viewport_width, viewport_height) = scene.selection.borrow().vp_size;
+                    let (viewport_width, viewport_height) = scene.selection.borrow().view.vp_size;
                     let aspect = if viewport_height > 0.0 {
                         viewport_width as f64 / viewport_height as f64
                     } else {