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
