Complete usenet download support for unarr CLI:
- NZB XML parser with password extraction from <head> meta
- yEnc decoder with CRC32 verification
- NNTP client with TLS, auth, and connection pool (up to 10 conns)
- Segment downloader with parallel workers and progress reporting
- Post-processing: par2 verify/repair, unrar/7z extraction with password support
- Agent client methods: SearchNzbs, DownloadNzb, GetUsenetCredentials
- UsenetDownloader implementing full Downloader interface
- Daemon wiring: UsenetDownloader passed to Manager
E2E tested: Oppenheimer 1080p (2.94 GB) downloaded via NNTP in 77.6s.
