feat(pipeline): integrate BorderMode and add UI preset

- params.py: border field now accepts bool | BorderMode
- presets.py: add UI_PRESET with BorderMode.UI, remove SIXEL_PRESET
- __init__.py: export UI_PRESET, drop SIXEL_PRESET
- registry.py: auto-register FrameBufferStage on discovery
- New FrameBufferStage for frame history and intensity maps
- Tests: update test_pipeline for UI preset, add test_framebuffer_stage.py

This sets the foundation for interactive UI panel and modern pipeline composition.

engine/pipeline/registry.py

@@ -118,6 +118,14 @@ def discover_stages() -> None:
     except ImportError:
         pass
 
+    # Register buffer stages (framebuffer, etc.)
+    try:
+        from engine.pipeline.stages.framebuffer import FrameBufferStage
+
+        StageRegistry.register("effect", FrameBufferStage)
+    except ImportError:
+        pass
+
     # Register display stages
     _register_display_stages()