Covers bad magic, unsupported version, wrong passphrase, truncation,
and tampered ciphertext. The wrong-passphrase / tampered-tag pair both
collapse to RelicarioError::Decrypt — same opaque-failure contract as
the live vault.
pack_backup / unpack_backup ship the magic header, format version,
Argon2id KDF, XChaCha20-Poly1305 AEAD, and zstd-compressed JSON
envelope. Empty-vault round-trip is the foundation; later tasks
add items, attachments, image, and git history.