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/data_sources/sources.py

@@ -124,7 +124,8 @@ class ListDataSource(DataSource):
     """
 
     def __init__(self, items, name: str = "list"):
-        self._items = items
+        self._raw_items = items  # Store raw items separately
+        self._items = None  # Cache for converted SourceItem objects
         self._name = name
 
     @property
@@ -138,7 +139,7 @@ class ListDataSource(DataSource):
     def fetch(self) -> list[SourceItem]:
         # Convert tuple items to SourceItem if needed
         result = []
-        for item in self._items:
+        for item in self._raw_items:
             if isinstance(item, SourceItem):
                 result.append(item)
             elif isinstance(item, tuple) and len(item) >= 3: