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)

engine/data_sources/sources.py

@@ -116,6 +116,44 @@ class EmptyDataSource(DataSource):
         return [SourceItem(content=content, source="empty", timestamp="0")]
 
 
+class ListDataSource(DataSource):
+    """Data source that wraps a pre-fetched list of items.
+
+    Used for bootstrap loading when items are already available in memory.
+    This is a simple wrapper for already-fetched data.
+    """
+
+    def __init__(self, items, name: str = "list"):
+        self._items = items
+        self._name = name
+
+    @property
+    def name(self) -> str:
+        return self._name
+
+    @property
+    def is_dynamic(self) -> bool:
+        return False
+
+    def fetch(self) -> list[SourceItem]:
+        # Convert tuple items to SourceItem if needed
+        result = []
+        for item in self._items:
+            if isinstance(item, SourceItem):
+                result.append(item)
+            elif isinstance(item, tuple) and len(item) >= 3:
+                # Assume (content, source, timestamp) tuple format
+                result.append(
+                    SourceItem(content=item[0], source=item[1], timestamp=str(item[2]))
+                )
+            else:
+                # Fallback: treat as string content
+                result.append(
+                    SourceItem(content=str(item), source="list", timestamp="0")
+                )
+        return result
+
+
 class PoetryDataSource(DataSource):
     """Data source for Poetry DB."""