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_framebuffer_stage.py

@@ -0,0 +1,236 @@
+"""
+Tests for FrameBufferStage.
+"""
+
+import pytest
+
+from engine.pipeline.core import DataType, PipelineContext
+from engine.pipeline.params import PipelineParams
+from engine.pipeline.stages.framebuffer import FrameBufferConfig, FrameBufferStage
+
+
+def make_ctx(width: int = 80, height: int = 24) -> PipelineContext:
+    """Create a PipelineContext for testing."""
+    ctx = PipelineContext()
+    params = PipelineParams()
+    params.viewport_width = width
+    params.viewport_height = height
+    ctx.params = params
+    return ctx
+
+
+class TestFrameBufferStage:
+    """Tests for FrameBufferStage."""
+
+    def test_init(self):
+        """FrameBufferStage initializes with default config."""
+        stage = FrameBufferStage()
+        assert stage.name == "framebuffer"
+        assert stage.category == "effect"
+        assert stage.config.history_depth == 2
+
+    def test_capabilities(self):
+        """Stage provides framebuffer.history capability."""
+        stage = FrameBufferStage()
+        assert "framebuffer.history" in stage.capabilities
+
+    def test_dependencies(self):
+        """Stage depends on render.output."""
+        stage = FrameBufferStage()
+        assert "render.output" in stage.dependencies
+
+    def test_inlet_outlet_types(self):
+        """Stage accepts and produces TEXT_BUFFER."""
+        stage = FrameBufferStage()
+        assert DataType.TEXT_BUFFER in stage.inlet_types
+        assert DataType.TEXT_BUFFER in stage.outlet_types
+
+    def test_init_context(self):
+        """init initializes context state."""
+        stage = FrameBufferStage()
+        ctx = make_ctx()
+
+        result = stage.init(ctx)
+
+        assert result is True
+        assert ctx.get("frame_history") == []
+        assert ctx.get("intensity_history") == []
+
+    def test_process_stores_buffer_in_history(self):
+        """process stores buffer in history."""
+        stage = FrameBufferStage()
+        ctx = make_ctx()
+        stage.init(ctx)
+
+        buffer = ["line1", "line2", "line3"]
+        result = stage.process(buffer, ctx)
+
+        assert result == buffer  # Pass-through
+        history = ctx.get("frame_history")
+        assert len(history) == 1
+        assert history[0] == buffer
+
+    def test_process_computes_intensity(self):
+        """process computes intensity map."""
+        stage = FrameBufferStage()
+        ctx = make_ctx()
+        stage.init(ctx)
+
+        buffer = ["hello world", "test line", ""]
+        stage.process(buffer, ctx)
+
+        intensity = ctx.get("current_intensity")
+        assert intensity is not None
+        assert len(intensity) == 3  # Three rows
+        # Non-empty lines should have intensity > 0
+        assert intensity[0] > 0
+        assert intensity[1] > 0
+        # Empty line should have intensity 0
+        assert intensity[2] == 0.0
+
+    def test_process_keeps_multiple_frames(self):
+        """process keeps configured depth of frames."""
+        config = FrameBufferConfig(history_depth=3)
+        stage = FrameBufferStage(config)
+        ctx = make_ctx()
+        stage.init(ctx)
+
+        # Process several frames
+        for i in range(5):
+            buffer = [f"frame {i}"]
+            stage.process(buffer, ctx)
+
+        history = ctx.get("frame_history")
+        assert len(history) == 3  # Only last 3 kept
+        # Should be in reverse chronological order (most recent first)
+        assert history[0] == ["frame 4"]
+        assert history[1] == ["frame 3"]
+        assert history[2] == ["frame 2"]
+
+    def test_process_keeps_intensity_sync(self):
+        """process keeps intensity history in sync with frame history."""
+        config = FrameBufferConfig(history_depth=3)
+        stage = FrameBufferStage(config)
+        ctx = make_ctx()
+        stage.init(ctx)
+
+        buffers = [
+            ["a"],
+            ["bb"],
+            ["ccc"],
+        ]
+        for buf in buffers:
+            stage.process(buf, ctx)
+
+        frame_hist = ctx.get("frame_history")
+        intensity_hist = ctx.get("intensity_history")
+        assert len(frame_hist) == len(intensity_hist) == 3
+
+        # Each frame's intensity should match
+        for i, frame in enumerate(frame_hist):
+            computed_intensity = stage._compute_buffer_intensity(frame, len(frame))
+            assert intensity_hist[i] == pytest.approx(computed_intensity)
+
+    def test_get_frame(self):
+        """get_frame retrieves frames from history by index."""
+        config = FrameBufferConfig(history_depth=3)
+        stage = FrameBufferStage(config)
+        ctx = make_ctx()
+        stage.init(ctx)
+
+        buffers = [["f1"], ["f2"], ["f3"]]
+        for buf in buffers:
+            stage.process(buf, ctx)
+
+        assert stage.get_frame(0, ctx) == ["f3"]  # Most recent
+        assert stage.get_frame(1, ctx) == ["f2"]
+        assert stage.get_frame(2, ctx) == ["f1"]
+        assert stage.get_frame(3, ctx) is None  # Out of range
+
+    def test_get_intensity(self):
+        """get_intensity retrieves intensity maps by index."""
+        stage = FrameBufferStage()
+        ctx = make_ctx()
+        stage.init(ctx)
+
+        buffers = [["line"], ["longer line"]]
+        for buf in buffers:
+            stage.process(buf, ctx)
+
+        intensity0 = stage.get_intensity(0, ctx)
+        intensity1 = stage.get_intensity(1, ctx)
+        assert intensity0 is not None
+        assert intensity1 is not None
+        # Longer line should have higher intensity (more non-space chars)
+        assert sum(intensity1) > sum(intensity0)
+
+    def test_compute_buffer_intensity_simple(self):
+        """_compute_buffer_intensity computes simple density."""
+        stage = FrameBufferStage()
+
+        buf = ["abc", "  ", "de"]
+        intensities = stage._compute_buffer_intensity(buf, max_rows=3)
+
+        assert len(intensities) == 3
+        # "abc" -> 3/3 = 1.0
+        assert pytest.approx(intensities[0]) == 1.0
+        # "  " -> 0/2 = 0.0
+        assert pytest.approx(intensities[1]) == 0.0
+        # "de" -> 2/2 = 1.0
+        assert pytest.approx(intensities[2]) == 1.0
+
+    def test_compute_buffer_intensity_with_ansi(self):
+        """_compute_buffer_intensity strips ANSI codes."""
+        stage = FrameBufferStage()
+
+        # Line with ANSI color codes
+        buf = ["\033[31mred\033[0m", "normal"]
+        intensities = stage._compute_buffer_intensity(buf, max_rows=2)
+
+        assert len(intensities) == 2
+        # Should treat "red" as 3 non-space chars
+        assert pytest.approx(intensities[0]) == 1.0  # "red" = 3/3
+        assert pytest.approx(intensities[1]) == 1.0  # "normal" = 6/6
+
+    def test_compute_buffer_intensity_padding(self):
+        """_compute_buffer_intensity pads to max_rows."""
+        stage = FrameBufferStage()
+
+        buf = ["short"]
+        intensities = stage._compute_buffer_intensity(buf, max_rows=5)
+
+        assert len(intensities) == 5
+        assert intensities[0] > 0
+        assert all(i == 0.0 for i in intensities[1:])
+
+    def test_thread_safety(self):
+        """process is thread-safe."""
+        from threading import Thread
+
+        stage = FrameBufferStage()
+        ctx = make_ctx()
+        stage.init(ctx)
+
+        results = []
+
+        def worker(idx):
+            buffer = [f"thread {idx}"]
+            stage.process(buffer, ctx)
+            results.append(len(ctx.get("frame_history", [])))
+
+        threads = [Thread(target=worker, args=(i,)) for i in range(10)]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join()
+
+        # All threads should see consistent state
+        assert len(ctx.get("frame_history")) <= 2  # Depth limit
+        # All worker threads should have completed without errors
+        assert len(results) == 10
+
+    def test_cleanup(self):
+        """cleanup does nothing but can be called."""
+        stage = FrameBufferStage()
+        # Should not raise
+        stage.cleanup()

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."""