refactor(remove): Delete RenderStage and ItemsStage classes (Phase 4.3)

- Delete RenderStage class (124 lines) - used legacy rendering
- Delete ItemsStage class (32 lines) - deprecated bootstrap mechanism
- Delete create_items_stage() function (3 lines)
- Add ListDataSource class to wrap pre-fetched items (38 lines)
- Update app.py to use ListDataSource + DataSourceStage instead of ItemsStage
- Remove deprecated test methods for RenderStage and ItemsStage
- Tests pass (508 core tests, legacy failures pre-existing)

engine/app.py

@@ -18,7 +18,6 @@ from engine.pipeline import (
 )
 from engine.pipeline.adapters import (
     SourceItemsToBufferStage,
-    create_items_stage,
     create_stage_from_display,
     create_stage_from_effect,
 )
@@ -147,7 +146,11 @@ def run_pipeline_mode(preset_name: str = "demo"):
         empty_source = EmptyDataSource(width=80, height=24)
         pipeline.add_stage("source", DataSourceStage(empty_source, name="empty"))
     else:
-        pipeline.add_stage("source", create_items_stage(items, preset.source))
+        from engine.data_sources.sources import ListDataSource
+        from engine.pipeline.adapters import DataSourceStage
+
+        list_source = ListDataSource(items, name=preset.source)
+        pipeline.add_stage("source", DataSourceStage(list_source, name=preset.source))
 
     # Add render stage - convert items to buffer
     pipeline.add_stage("render", SourceItemsToBufferStage(name="items-to-buffer"))