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/presets.py

@@ -13,6 +13,7 @@ Loading order:
 from dataclasses import dataclass, field
 from typing import Any
 
+from engine.display import BorderMode
 from engine.pipeline.params import PipelineParams
 
 
@@ -26,7 +27,6 @@ def _load_toml_presets() -> dict[str, Any]:
         return {}
 
 
-# Pre-load TOML presets
 _YAML_PRESETS = _load_toml_presets()
 
 
@@ -47,14 +47,24 @@ class PipelinePreset:
     display: str = "terminal"
     camera: str = "scroll"
     effects: list[str] = field(default_factory=list)
-    border: bool = False
+    border: bool | BorderMode = (
+        False  # Border mode: False=off, True=simple, BorderMode.UI for panel
+    )
 
     def to_params(self) -> PipelineParams:
         """Convert to PipelineParams."""
+        from engine.display import BorderMode
+
         params = PipelineParams()
         params.source = self.source
         params.display = self.display
-        params.border = self.border
+        params.border = (
+            self.border
+            if isinstance(self.border, bool)
+            else BorderMode.UI
+            if self.border == BorderMode.UI
+            else False
+        )
         params.camera_mode = self.camera
         params.effect_order = self.effects.copy()
         return params
@@ -83,6 +93,16 @@ DEMO_PRESET = PipelinePreset(
     effects=["noise", "fade", "glitch", "firehose"],
 )
 
+UI_PRESET = PipelinePreset(
+    name="ui",
+    description="Interactive UI mode with right-side control panel",
+    source="fixture",
+    display="pygame",
+    camera="scroll",
+    effects=["noise", "fade", "glitch"],
+    border=BorderMode.UI,
+)
+
 POETRY_PRESET = PipelinePreset(
     name="poetry",
     description="Poetry feed with subtle effects",
@@ -110,15 +130,6 @@ WEBSOCKET_PRESET = PipelinePreset(
     effects=["noise", "fade", "glitch"],
 )
 
-SIXEL_PRESET = PipelinePreset(
-    name="sixel",
-    description="Sixel graphics display mode",
-    source="headlines",
-    display="sixel",
-    camera="scroll",
-    effects=["noise", "fade", "glitch"],
-)
-
 FIREHOSE_PRESET = PipelinePreset(
     name="firehose",
     description="High-speed firehose mode",
@@ -128,6 +139,16 @@ FIREHOSE_PRESET = PipelinePreset(
     effects=["noise", "fade", "glitch", "firehose"],
 )
 
+FIXTURE_PRESET = PipelinePreset(
+    name="fixture",
+    description="Use cached headline fixtures",
+    source="fixture",
+    display="pygame",
+    camera="scroll",
+    effects=["noise", "fade"],
+    border=False,
+)
+
 
 # Build presets from YAML data
 def _build_presets() -> dict[str, PipelinePreset]:
@@ -145,8 +166,9 @@ def _build_presets() -> dict[str, PipelinePreset]:
         "poetry": POETRY_PRESET,
         "pipeline": PIPELINE_VIZ_PRESET,
         "websocket": WEBSOCKET_PRESET,
-        "sixel": SIXEL_PRESET,
         "firehose": FIREHOSE_PRESET,
+        "ui": UI_PRESET,
+        "fixture": FIXTURE_PRESET,
     }
 
     for name, preset in builtins.items():