Borgbackup Pack Files for borg2

Borg stores each deduplicated chunk as a separate file. On SFTP, that's one network round-trip per chunk. Pack files bundle N chunks into one file. A TOC sits in the trailer, and the pack gets a SHA256 derived pack_id. The client caches a small index per pack locally. Borg compact handles cleanup by mark-and-sweep to find dead chunks, then rewrite sparse packs into dense ones. Deliverables: pack file write and read support in borg2, borg compact with pack rewriting, incremental per-pack index cache, and encrypted pack support.