feat: integrate SimpleCADAPI 2.0.2 CAD workflows

SimpleCADAPI/docs/api/export_model_json.md

@@ -3,7 +3,12 @@
 ## API Definition
 
 ```python
-def export_model_json(session: 'GraphSession', indent: int = 2) -> str
+def export_model_json(
+    session: 'GraphSession',
+    indent: int = 2,
+    *,
+    result_node_ids: Optional[Sequence[str]] = None,
+) -> str
 ```
 
 *Source: serializer.py*
@@ -20,3 +25,8 @@ Current Phase 1 scope uses the active session as the container of:
 - operation graph
 - expression graph
 - capabilities/schema metadata
+
+When `result_node_ids` is omitted, explicitly captured session results are used
+when available; otherwise export falls back to graph leaves. New top-level model
+code normally reads `ModelResult.model_json` instead of calling this function
+directly.