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

engine/fixtures/headlines.json

@@ -0,0 +1,19 @@
+{
+  "items": [
+    ["Breaking: AI systems achieve breakthrough in natural language understanding", "TechDaily", "14:32"],
+    ["Scientists discover new exoplanet in habitable zone", "ScienceNews", "13:15"],
+    ["Global markets rally as inflation shows signs of cooling", "FinanceWire", "12:48"],
+    ["New study reveals benefits of Mediterranean diet for cognitive health", "HealthJournal", "11:22"],
+    ["Tech giants announce collaboration on AI safety standards", "TechDaily", "10:55"],
+    ["Archaeologists uncover 3000-year-old city in desert", "HistoryNow", "09:30"],
+    ["Renewable energy capacity surpasses fossil fuels for first time", "GreenWorld", "08:15"],
+    ["Space agency prepares for next Mars mission launch window", "SpaceNews", "07:42"],
+    ["New film breaks box office records on opening weekend", "EntertainmentHub", "06:18"],
+    ["Local community raises funds for new library project", "CommunityPost", "05:30"],
+    ["Quantum computing breakthrough could revolutionize cryptography", "TechWeekly", "15:20"],
+    ["New species of deep-sea creature discovered in Pacific trench", "NatureToday", "14:05"],
+    ["Electric vehicle sales surpass traditional cars in Europe", "AutoNews", "12:33"],
+    ["Renowned artist unveils interactive AI-generated exhibition", "ArtsMonthly", "11:10"],
+    ["Climate summit reaches historic agreement on emissions", "WorldNews", "09:55"]
+  ]
+}