feat(core): add derive_master_key_raw + RecoveryQr error variant

crates/relicario-core/src/crypto.rs

@@ -243,6 +243,23 @@ pub fn derive_master_key(
     Ok(output)
 }
 
+/// Like `derive_master_key` but takes an already-assembled `input` byte slice directly,
+/// allowing callers to apply their own domain separation before KDF.
+pub fn derive_master_key_raw(
+    input: &[u8],
+    salt: &[u8; 32],
+    params: &KdfParams,
+) -> Result<Zeroizing<[u8; 32]>> {
+    let argon2_params = Params::new(params.argon2_m, params.argon2_t, params.argon2_p, Some(32))
+        .map_err(|e| RelicarioError::Kdf(e.to_string()))?;
+    let argon2 = Argon2::new(Algorithm::Argon2id, Version::V0x13, argon2_params);
+    let mut output = Zeroizing::new([0u8; 32]);
+    argon2
+        .hash_password_into(input, salt, output.as_mut())
+        .map_err(|e| RelicarioError::Kdf(e.to_string()))?;
+    Ok(output)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

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.