feat(app): add direct CLI mode, validation framework, fixtures, and UI panel integration

- Add run_pipeline_mode_direct() for constructing pipelines from CLI flags
- Add engine/pipeline/validation.py with validate_pipeline_config() and MVP rules
- Add fixtures system: engine/fixtures/headlines.json for cached test data
- Enhance fetch.py to use fixtures cache path
- Support fixture source in run_pipeline_mode()
- Add --pipeline-* CLI flags: source, effects, camera, display, UI, border
- Integrate UIPanel: raw mode, preset picker, event callbacks, param adjustment
- Add UI_PRESET support in app and hot-rebuild pipeline on preset change
- Add test UIPanel rendering and interaction tests

This provides a flexible pipeline construction interface with validation and interactive control.

Fixes #29, #30, #31

test_ui_simple.py

@@ -0,0 +1,56 @@
+"""
+Simple test for UIPanel integration.
+"""
+
+from engine.pipeline.ui import UIPanel, UIConfig, StageControl
+
+# Create panel
+panel = UIPanel(UIConfig(panel_width=24))
+
+# Add some mock stages
+panel.register_stage(
+    type(
+        "Stage", (), {"name": "noise", "category": "effect", "is_enabled": lambda: True}
+    ),
+    enabled=True,
+)
+panel.register_stage(
+    type(
+        "Stage", (), {"name": "fade", "category": "effect", "is_enabled": lambda: True}
+    ),
+    enabled=False,
+)
+panel.register_stage(
+    type(
+        "Stage",
+        (),
+        {"name": "glitch", "category": "effect", "is_enabled": lambda: True},
+    ),
+    enabled=True,
+)
+panel.register_stage(
+    type(
+        "Stage",
+        (),
+        {"name": "font", "category": "transform", "is_enabled": lambda: True},
+    ),
+    enabled=True,
+)
+
+# Select first stage
+panel.select_stage("noise")
+
+# Render at 80x24
+lines = panel.render(80, 24)
+print("\n".join(lines))
+
+print("\nStage list:")
+for name, ctrl in panel.stages.items():
+    print(f"  {name}: enabled={ctrl.enabled}, selected={ctrl.selected}")
+
+print("\nToggle 'fade' and re-render:")
+panel.toggle_stage("fade")
+lines = panel.render(80, 24)
+print("\n".join(lines))
+
+print("\nEnabled stages:", panel.get_enabled_stages())