feat(core): add derive_master_key_raw + RecoveryQr error variant

crates/relicario-core/src/error.rs

@@ -119,6 +119,10 @@ pub enum RelicarioError {
     /// immediately. Use TOTP instead.
     #[error("HOTP is not supported: counter persistence requires vault save after each use")]
     HotpNotSupported,
+
+    /// Recovery QR generation or parsing failed.
+    #[error("recovery QR: {0}")]
+    RecoveryQr(String),
 }
 
 /// Crate-wide result alias, reducing boilerplate in function signatures.