fix: ListDataSource cache and camera dependency resolution

Two critical fixes:

1. ListDataSource Cache Bug
   - Previously, ListDataSource.__init__ cached raw tuples directly
   - get_items() would return cached raw tuples without converting to SourceItem
   - This caused SourceItemsToBufferStage to receive tuples and stringify them
   - Results: ugly tuple representations in terminal/pygame instead of formatted text
   - Fix: Store raw items in _raw_items, let fetch() convert to SourceItem
   - Cache now contains proper SourceItem objects

2. Camera Dependency Resolution
   - CameraStage declared dependency on 'source.items' exactly
   - DataSourceStage provides 'source.headlines' (or 'source.poetry', etc.)
   - Capability matching didn't trigger prefix match for exact dependency
   - Fix: Change CameraStage dependency to 'source' for prefix matching

3. Added app.py Camera Stage Support
   - Pipeline now adds camera stage from preset.camera config
   - Supports vertical, horizontal, omni, floating, bounce modes
   - Tests now passing with proper data flow through all stages

Tests: All 502 tests passing, 16 skipped

engine/effects/__init__.py

@@ -18,13 +18,6 @@ from engine.effects.types import (
     create_effect_context,
 )
 
-
-def get_effect_chain():
-    from engine.legacy.layers import get_effect_chain as _chain
-
-    return _chain()
-
-
 __all__ = [
     "EffectChain",
     "EffectRegistry",
@@ -34,7 +27,6 @@ __all__ = [
     "create_effect_context",
     "get_registry",
     "set_registry",
-    "get_effect_chain",
     "get_monitor",
     "set_monitor",
     "PerformanceMonitor",