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)

tests/test_pipeline.py

@@ -586,47 +586,6 @@ class TestPipelinePresets:
 class TestStageAdapters:
     """Tests for pipeline stage adapters."""
 
-    def test_render_stage_capabilities(self):
-        """RenderStage declares correct capabilities."""
-        from engine.pipeline.adapters import RenderStage
-
-        stage = RenderStage(items=[], name="render")
-        assert "render.output" in stage.capabilities
-
-    def test_render_stage_dependencies(self):
-        """RenderStage declares correct dependencies."""
-        from engine.pipeline.adapters import RenderStage
-
-        stage = RenderStage(items=[], name="render")
-        assert "source" in stage.dependencies
-
-    def test_render_stage_process(self):
-        """RenderStage.process returns buffer."""
-        from engine.pipeline.adapters import RenderStage
-        from engine.pipeline.core import PipelineContext
-
-        items = [
-            ("Test Headline", "test", 1234567890.0),
-        ]
-        stage = RenderStage(items=items, width=80, height=24)
-        ctx = PipelineContext()
-
-        result = stage.process(None, ctx)
-        assert result is not None
-        assert isinstance(result, list)
-
-    def test_items_stage(self):
-        """ItemsStage provides items to pipeline."""
-        from engine.pipeline.adapters import ItemsStage
-        from engine.pipeline.core import PipelineContext
-
-        items = [("Headline 1", "src1", 123.0), ("Headline 2", "src2", 124.0)]
-        stage = ItemsStage(items, name="headlines")
-        ctx = PipelineContext()
-
-        result = stage.process(None, ctx)
-        assert result == items
-
     def test_display_stage_init(self):
         """DisplayStage.init initializes display."""
         from engine.display.backends.null import NullDisplay
@@ -765,55 +724,6 @@ class TestEffectPluginStage:
 class TestFullPipeline:
     """End-to-end tests for the full pipeline."""
 
-    def test_pipeline_with_items_and_effect(self):
-        """Pipeline executes items->effect flow."""
-        from engine.effects.types import EffectConfig, EffectPlugin
-        from engine.pipeline.adapters import EffectPluginStage, ItemsStage
-        from engine.pipeline.controller import Pipeline, PipelineConfig
-
-        class TestEffect(EffectPlugin):
-            name = "test"
-            config = EffectConfig()
-
-            def process(self, buf, ctx):
-                return [f"processed: {line}" for line in buf]
-
-            def configure(self, config):
-                pass
-
-        pipeline = Pipeline(config=PipelineConfig(enable_metrics=False))
-
-        # Items stage
-        items = [("Headline 1", "src1", 123.0)]
-        pipeline.add_stage("source", ItemsStage(items, name="headlines"))
-
-        # Effect stage
-        pipeline.add_stage("effect", EffectPluginStage(TestEffect(), name="test"))
-
-        pipeline.build()
-
-        result = pipeline.execute(None)
-        assert result.success is True
-        assert "processed:" in result.data[0]
-
-    def test_pipeline_with_items_stage(self):
-        """Pipeline with ItemsStage provides items through pipeline."""
-        from engine.pipeline.adapters import ItemsStage
-        from engine.pipeline.controller import Pipeline, PipelineConfig
-
-        pipeline = Pipeline(config=PipelineConfig(enable_metrics=False))
-
-        # Items stage provides source
-        items = [("Headline 1", "src1", 123.0), ("Headline 2", "src2", 124.0)]
-        pipeline.add_stage("source", ItemsStage(items, name="headlines"))
-
-        pipeline.build()
-
-        result = pipeline.execute(None)
-        assert result.success is True
-        # Items are passed through
-        assert result.data == items
-
     def test_pipeline_circular_dependency_detection(self):
         """Pipeline detects circular dependencies."""
         from engine.pipeline.controller import Pipeline
@@ -857,33 +767,6 @@ class TestFullPipeline:
         except Exception:
             pass
 
-    def test_datasource_stage_capabilities_match_render_deps(self):
-        """DataSourceStage provides capability that RenderStage can depend on."""
-        from engine.data_sources.sources import HeadlinesDataSource
-        from engine.pipeline.adapters import DataSourceStage, RenderStage
-
-        # DataSourceStage provides "source.headlines"
-        ds_stage = DataSourceStage(HeadlinesDataSource(), name="headlines")
-        assert "source.headlines" in ds_stage.capabilities
-
-        # RenderStage depends on "source"
-        r_stage = RenderStage(items=[], width=80, height=24)
-        assert "source" in r_stage.dependencies
-
-        # Test the capability matching directly
-        from engine.pipeline.controller import Pipeline, PipelineConfig
-
-        pipeline = Pipeline(config=PipelineConfig(enable_metrics=False))
-        pipeline.add_stage("source", ds_stage)
-        pipeline.add_stage("render", r_stage)
-
-        # Build capability map and test matching
-        pipeline._capability_map = pipeline._build_capability_map()
-
-        # "source" should match "source.headlines"
-        match = pipeline._find_stage_with_capability("source")
-        assert match == "source", f"Expected 'source', got {match}"
-
 
 class TestPipelineMetrics:
     """Tests for pipeline metrics collection."""