- Pack signature accepts optional salt, writes 16-byte salt between header and TOC
- Set flags bit 4 and adjust toc_offset to 56 when salt present
- read_archive_metadata returns salt alongside header and TOC entries
- Add read_archive_salt() public helper for pre-unpack salt reading
- main.rs uses resolve_key_for_pack/resolve_key_for_unpack for two-phase password flow
- Add 5 new integration tests: password roundtrip, wrong password rejection,
salt flag presence, no-salt flag for key archives, directory password roundtrip
- All 52 tests pass (25 unit + 7 golden + 20 integration)