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.

tests/test_pipeline.py

@@ -45,7 +45,8 @@ class TestStageRegistry:
         assert "pygame" in displays
         assert "websocket" in displays
         assert "null" in displays
-        assert "sixel" in displays
+        # sixel and kitty removed; should not be present
+        assert "sixel" not in displays
 
     def test_create_source_stage(self):
         """StageRegistry.create creates source stages."""
@@ -546,7 +547,7 @@ class TestPipelinePresets:
             FIREHOSE_PRESET,
             PIPELINE_VIZ_PRESET,
             POETRY_PRESET,
-            SIXEL_PRESET,
+            UI_PRESET,
             WEBSOCKET_PRESET,
         )
 
@@ -554,8 +555,8 @@ class TestPipelinePresets:
         assert POETRY_PRESET.name == "poetry"
         assert FIREHOSE_PRESET.name == "firehose"
         assert PIPELINE_VIZ_PRESET.name == "pipeline"
-        assert SIXEL_PRESET.name == "sixel"
         assert WEBSOCKET_PRESET.name == "websocket"
+        assert UI_PRESET.name == "ui"
 
     def test_preset_to_params(self):
         """Presets convert to PipelineParams correctly."""