ocs_doc_api: Phase 4 (paper-space viewports)

- Operation::CreateViewport(ViewportSpec) appended to the wire enum (append-only).
- backend add_viewport (center/width/height/view_target/view_height).
- Viewport bounds (center +- w/2,h/2), transform (moves center), ensure_transformable.
- facade EntityCollection::create_viewport; spec Viewport family; api_reference +
  binding_schema regenerated; mocks updated.
- Test phase4_create_viewport_bounds_transform -> 600 host lib green.
- IMPLEMENTATION.md: Phase 4 done (set_view/view-query remaining).

crates/ocs_doc_api/src/gen/binding_schema.json

@@ -194,6 +194,13 @@
       "handle": "Entity",
       "methods": [],
       "name": "Insert"
+    },
+    {
+      "acadrust_variant": "Viewport",
+      "collection": "entities",
+      "handle": "Entity",
+      "methods": [],
+      "name": "Viewport"
     }
   ],
   "layouts": {