fix: DisplayStage dependency and render pipeline data flow

Critical fix for display rendering:

1. DisplayStage Missing Dependency
   - DisplayStage had empty dependencies, causing it to execute before render
   - No data was reaching the display output
   - Fix: Add 'render.output' dependency so display comes after render stages
   - Now proper execution order: source → render → display

2. create_default_pipeline Missing Render Stage
   - Default pipeline only had source and display, no render stage between
   - Would fail validation with 'Missing render.output' capability
   - Fix: Add SourceItemsToBufferStage to convert items to text buffer
   - Now complete data flow: source → render → display

3. Updated Test Expectations
   - test_display_stage_dependencies now expects 'render.output' dependency

Result: Display backends (pygame, terminal, websocket) now receive proper
rendered text buffers and can display output correctly.

Tests: All 502 tests passing

tests/test_adapters.py

@@ -97,10 +97,10 @@ class TestDisplayStage:
         assert "display.output" in stage.capabilities
 
     def test_display_stage_dependencies(self):
-        """DisplayStage has no dependencies."""
+        """DisplayStage depends on render.output."""
         mock_display = MagicMock()
         stage = DisplayStage(mock_display, name="terminal")
-        assert stage.dependencies == set()
+        assert "render.output" in stage.dependencies
 
     def test_display_stage_init(self):
         """DisplayStage.init() calls display.init() with dimensions."""