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/params.py

@@ -8,6 +8,11 @@ modify these params, which the pipeline then applies to its stages.
 from dataclasses import dataclass, field
 from typing import Any
 
+try:
+    from engine.display import BorderMode
+except ImportError:
+    BorderMode = object  # Fallback for type checking
+
 
 @dataclass
 class PipelineParams:
@@ -23,7 +28,7 @@ class PipelineParams:
 
     # Display config
     display: str = "terminal"
-    border: bool = False
+    border: bool | BorderMode = False
 
     # Camera config
     camera_mode: str = "vertical"