refactor(remove-renderstage): Remove RenderStage usage from app.py (Phase 4.2)

- Remove RenderStage import from engine/app.py
- Replace RenderStage with SourceItemsToBufferStage for all sources
- Simplifies render pipeline - no more special-case logic
- SourceItemsToBufferStage properly converts items to text buffer
- Tests pass (11 app tests)

engine/app.py

@@ -17,7 +17,6 @@ from engine.pipeline import (
     list_presets,
 )
 from engine.pipeline.adapters import (
-    RenderStage,
     SourceItemsToBufferStage,
     create_items_stage,
     create_stage_from_display,
@@ -150,21 +149,8 @@ def run_pipeline_mode(preset_name: str = "demo"):
     else:
         pipeline.add_stage("source", create_items_stage(items, preset.source))
 
-    # Add appropriate render stage
+    # Add render stage - convert items to buffer
-    if preset.source in ("pipeline-inspect", "empty"):
     pipeline.add_stage("render", SourceItemsToBufferStage(name="items-to-buffer"))
-    else:
-        pipeline.add_stage(
-            "render",
-            RenderStage(
-                items,
-                width=80,
-                height=24,
-                camera_speed=params.camera_speed,
-                camera_mode=preset.camera,
-                firehose_enabled=params.firehose_enabled,
-            ),
-        )
 
     for effect_name in preset.effects:
         effect = effect_registry.get(effect_name)