- Add argon2 0.5 and rpassword 7.4 dependencies
- Implement derive_key_from_password() using Argon2id with 16-byte salt
- Implement prompt_password() with optional confirmation for pack
- Add resolve_key_for_pack() (generates random salt) and resolve_key_for_unpack() (reads salt from archive)
- Add FLAG_KDF_SALT (bit 4), SALT_SIZE constant, read_salt/write_salt functions to format.rs
- Relax flags validation to allow bit 4 (bits 5-7 must be zero)
- Create src/lib.rs re-exporting all modules as public
- Update main.rs to use encrypted_archive:: imports instead of local mod declarations
- Add dev-dependencies: tempfile, assert_cmd, hex-literal, predicates
