feat(core): backup module — empty-vault round-trip

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.

crates/relicario-core/src/lib.rs

@@ -77,3 +77,6 @@ pub use vault::{
 };
 
 pub mod imgsecret;
+
+pub mod backup;
+pub use backup::{pack_backup, unpack_backup, BackupInput, BackupOutput, BackupItem, BackupAttachment};