- Extract SubtitleWriter classes (SrtWriter, AssWriter, VttWriter) to infrastructure/subtitle_writer.py
- Add create_subtitle_writer() factory function
- Remove old SubtitleWriter class and _format_timestamp from conversion_runner.py
- Use create_subtitle_writer() factory from infrastructure layer
- Add tests/test_subtitle_writer.py with 28 tests covering SrtWriter, AssWriter, VttWriter
- All tests match old _format_timestamp behavior