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/fetch.py

@@ -117,11 +117,12 @@ def fetch_poetry():
 
 
 # ─── CACHE ────────────────────────────────────────────────
-_CACHE_DIR = pathlib.Path(__file__).resolve().parent.parent
+# Cache moved to engine/fixtures/headlines.json
+_CACHE_DIR = pathlib.Path(__file__).resolve().parent / "fixtures"
 
 
 def _cache_path():
-    return _CACHE_DIR / f".mainline_cache_{config.MODE}.json"
+    return _CACHE_DIR / "headlines.json"
 
 
 def load_cache():