refactor(remove): Remove RenderStage and ItemsStage from pipeline.py introspection (Phase 4.4)

- Remove ItemsStage documentation entry from introspection
- Remove RenderStage documentation entry from introspection
- Keep remaining adapter documentation up to date
- Tests pass (508 core tests)

engine/pipeline.py

@@ -483,18 +483,6 @@ class PipelineIntrospector:
         """
 
         stages_info = [
-            (
-                "ItemsSource",
-                "engine.pipeline.adapters",
-                "ItemsStage",
-                "Provides pre-fetched items",
-            ),
-            (
-                "Render",
-                "engine.pipeline.adapters",
-                "RenderStage",
-                "Renders items to buffer",
-            ),
             (
                 "Effect",
                 "engine.pipeline.adapters",