style: Fix linting errors and organize imports

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/app/main.py

@@ -10,12 +10,18 @@ from engine.display import BorderMode, DisplayRegistry
 from engine.effects import get_registry
 from engine.fetch import fetch_all, fetch_all_fast, fetch_poetry, load_cache, save_cache
 
+# Import from engine (Mainline-specific)
+from engine.pipeline import list_presets
+
+# Import from engine (Mainline-specific)
+from engine.pipeline.ui import UIConfig, UIPanel
+from engine.pipeline.validation import validate_pipeline_config
+
 # Import from sideline (the framework)
 from sideline.pipeline import (
     Pipeline,
     PipelineConfig,
     PipelineContext,
-    StageRegistry,
 )
 from sideline.pipeline.adapters import (
     CameraStage,
@@ -26,10 +32,6 @@ from sideline.pipeline.adapters import (
 )
 from sideline.pipeline.params import PipelineParams
 
-# Import from engine (Mainline-specific)
-from engine.pipeline.ui import UIConfig, UIPanel
-from engine.pipeline.validation import validate_pipeline_config
-
 try:
     from engine.display.backends.websocket import WebSocketDisplay
 except ImportError:

engine/pipeline/__init__.py

@@ -9,28 +9,6 @@ Note: This module is deprecated and will be removed in future versions.
 """
 
 # Re-export from sideline for backward compatibility
-from sideline.pipeline import (
-    Pipeline,
-    PipelineConfig,
-    PipelineContext,
-    Stage,
-    StageConfig,
-    StageError,
-    StageResult,
-    PipelineParams,
-    StageRegistry,
-    discover_stages,
-    register_camera,
-    register_display,
-    register_effect,
-    register_source,
-)
-
-# Also re-export from sideline.core for compatibility
-from sideline.pipeline.core import (
-    DataType,
-)
-
 # Re-export from engine.pipeline.presets (Mainline-specific)
 from engine.pipeline.presets import (
     DEMO_PRESET,
@@ -45,6 +23,32 @@ from engine.pipeline.presets import (
     list_presets,
 )
 
+# Re-export additional functions from sideline.pipeline
+from sideline.pipeline import (
+    Pipeline,
+    PipelineConfig,
+    PipelineContext,
+    PipelineParams,
+    PipelineRunner,
+    Stage,
+    StageConfig,
+    StageError,
+    StageRegistry,
+    StageResult,
+    create_default_pipeline,
+    create_pipeline_from_params,
+    discover_stages,
+    register_camera,
+    register_display,
+    register_effect,
+    register_source,
+)
+
+# Also re-export from sideline.core for compatibility
+from sideline.pipeline.core import (
+    DataType,
+)
+
 # Re-export from sideline.pipeline.params
 from sideline.pipeline.params import (
     DEFAULT_HEADLINE_PARAMS,
@@ -52,13 +56,6 @@ from sideline.pipeline.params import (
     DEFAULT_PYGAME_PARAMS,
 )
 
-# Re-export additional functions from sideline.pipeline
-from sideline.pipeline import (
-    create_default_pipeline,
-    create_pipeline_from_params,
-    PipelineRunner,
-)
-
 __all__ = [
     # Core (from sideline)
     "Stage",

engine/plugins.py

@@ -38,10 +38,10 @@ def register_stages(registry):
 def _register_data_sources(registry):
     """Register Mainline data source stages."""
     try:
-        from engine.data_sources.sources import HeadlinesDataSource, PoetryDataSource
         from engine.data_sources.pipeline_introspection import (
             PipelineIntrospectionSource,
         )
+        from engine.data_sources.sources import HeadlinesDataSource, PoetryDataSource
 
         registry.register("source", HeadlinesDataSource)
         registry.register("source", PoetryDataSource)
@@ -60,13 +60,6 @@ def _register_data_sources(registry):
 def _register_effects(registry):
     """Register Mainline effect stages."""
     try:
-        # Register effects
-        from sideline.effects import EffectRegistry
-        from sideline.effects.registry import get_registry
-
-        # Get the global effect registry instance
-        effect_registry = get_registry()
-
         # Note: EffectRegistry stores effect instances, not classes
         # For now, skip effect registration since it requires more refactoring
         logger.info("Effect registration skipped (requires effect refactoring)")