feat(core): key-file armor (relicario-keyfile-v1) encode/decode

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01McMF5pUJbCMricmFEnf2sG

crates/relicario-core/src/lib.rs

@@ -84,6 +84,8 @@ pub use vault::{
 
 pub mod imgsecret;
 
+pub mod keyfile;
+
 pub mod backup;
 pub use backup::{pack_backup, unpack_backup, BackupInput, BackupOutput, BackupItem, BackupAttachment};