fix: Update imports to use engine.pipeline instead of engine.pipeline.core

The old engine/pipeline/core.py file was removed as part of the Sideline/Mainline split.
All imports that referenced engine.pipeline.core have been updated to use engine.pipeline
which re-exports from sideline.pipeline.core.

This ensures consistency and avoids duplicate DataType enum instances.

engine/pipeline/adapters/display.py

@@ -2,7 +2,7 @@
 
 from typing import Any
 
-from engine.pipeline.core import PipelineContext, Stage
+from engine.pipeline import PipelineContext, Stage
 
 
 class DisplayStage(Stage):
@@ -59,13 +59,13 @@ class DisplayStage(Stage):
 
     @property
     def inlet_types(self) -> set:
-        from engine.pipeline.core import DataType
+        from engine.pipeline import DataType
 
         return {DataType.TEXT_BUFFER}  # Display consumes rendered text
 
     @property
     def outlet_types(self) -> set:
-        from engine.pipeline.core import DataType
+        from engine.pipeline import DataType
 
         return {DataType.NONE}  # Display is a terminal stage (no output)