refactor: extract config.py, add burn_only, fix title_segments and font size

- Extract all path/API config to config.py (single source of truth)
- Add run.py / burn_only.py / run.bat / burn.bat entry points
- burn_only: skip transcription/subtitle gen, fast reburn existing SRTs
- Fix title_segments: use transcript keyword time for split point
- Fix subtitle: each overlapping title shows max title_duration (not full clip)
- Fix burn_only font size: default from 90 to 60
- Delete old run_lesson1.bat/py, temp debug scripts
- Update README, ARCHITECTURE, CHANGELOG, add USAGE.md

src/core/constants.py

@@ -79,7 +79,7 @@ DEFAULT_OUTPUT_DIR = os.path.join(PROJECT_ROOT, "output")
 DEFAULT_VIDEO_PARAMS = {
     "fade_duration": 1,
     "title_duration": 3,
-    "title_fontsize": 90,
+    "title_fontsize": 60,
     "title_color": "FFFF00",
     "subtitle_fontsize": 24,
     "subtitle_color": "FFFFFF",