Add render reference asset manifest contract

src/markitect_tool/extension/builtins.py

@@ -8,6 +8,7 @@ from markitect_tool.query import default_query_engine_registry
 from markitect_tool.render import (
     default_render_export_adapter_registry,
     render_export_registry_descriptor,
+    render_reference_manifest_descriptor,
 )
 from markitect_tool.source import (
     default_source_adapter_registry,
@@ -33,6 +34,7 @@ def builtin_extension_registry() -> ExtensionRegistry:
         _memory_runtime_adapter_descriptor(),
         _agent_memory_descriptor(),
         render_export_registry_descriptor(),
+        render_reference_manifest_descriptor(),
         source_adapter_registry_descriptor(),
     ]:
         registry.register(descriptor)