TDD GREEN phase:
- DownloadProcessor inherits from BaseProcessor
- Fetches metadata from Bilibili and Zhihu adapters
- Saves metadata to content_dir as JSON
- Records state on success/failure
- Skips step if already completed (idempotency)
- All metadata tests pass
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
