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/__init__.py

@@ -50,8 +50,7 @@ from engine.pipeline.presets import (
     FIREHOSE_PRESET,
     PIPELINE_VIZ_PRESET,
     POETRY_PRESET,
-    PRESETS,
-    SIXEL_PRESET,
+    UI_PRESET,
     WEBSOCKET_PRESET,
     PipelinePreset,
     create_preset_from_params,
@@ -92,8 +91,8 @@ __all__ = [
     "POETRY_PRESET",
     "PIPELINE_VIZ_PRESET",
     "WEBSOCKET_PRESET",
-    "SIXEL_PRESET",
     "FIREHOSE_PRESET",
+    "UI_PRESET",
     "get_preset",
     "list_presets",
     "create_preset_from_params",