fix(core): disable HOTP with clear error (audit I6)

HOTP requires incrementing and persisting the counter after each use.
Without vault-save machinery in compute_totp_code, HOTP would desync
immediately. Now returns HotpNotSupported error.

TOTP and Steam codes continue to work.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

crates/relicario-core/src/error.rs

@@ -109,6 +109,12 @@ pub enum RelicarioError {
     /// rotating the passphrase or reference image.
     #[error("device key error: {0}")]
     DeviceKey(String),
+
+    /// HOTP requires incrementing and persisting the counter after each use.
+    /// Without vault-save machinery in compute_totp_code, HOTP would desync
+    /// immediately. Use TOTP instead.
+    #[error("HOTP is not supported: counter persistence requires vault save after each use")]
+    HotpNotSupported,
 }
 
 /// Crate-wide result alias, reducing boilerplate in function signatures.

crates/relicario-core/src/item_types/totp.rs

@@ -64,14 +64,14 @@ impl Default for TotpKind {
     fn default() -> Self { TotpKind::Totp }
 }
 
-/// Compute a TOTP/HOTP/Steam code for `config` at the given Unix timestamp.
+/// Compute a TOTP/Steam code for `config` at the given Unix timestamp.
 ///
 /// For TOTP and Steam: counter = `now_unix_seconds / period_seconds`.
-/// For HOTP: uses the `counter` carried in the variant.
+/// HOTP is not supported — returns [`RelicarioError::HotpNotSupported`].
 pub fn compute_totp_code(config: &TotpConfig, now_unix_seconds: u64) -> Result<String> {
     let counter = match config.kind {
         TotpKind::Totp => now_unix_seconds / config.period_seconds as u64,
-        TotpKind::Hotp { counter } => counter,
+        TotpKind::Hotp { .. } => return Err(RelicarioError::HotpNotSupported),
         TotpKind::Steam => now_unix_seconds / config.period_seconds as u64,
     };
     let counter_bytes = counter.to_be_bytes();
@@ -165,7 +165,7 @@ mod tests {
     }
 
     #[test]
-    fn hotp_carries_counter() {
+    fn hotp_kind_roundtrips_through_json() {
         let cfg = TotpConfig { kind: TotpKind::Hotp { counter: 42 }, ..TotpConfig::default() };
         let json = serde_json::to_string(&cfg).unwrap();
         let parsed: TotpConfig = serde_json::from_str(&json).unwrap();
@@ -173,6 +173,18 @@ mod tests {
             TotpKind::Hotp { counter } => assert_eq!(counter, 42),
             other => panic!("expected Hotp, got {:?}", other),
         }
+        // Note: compute_totp_code will reject this — HOTP not supported
+    }
+
+    #[test]
+    fn hotp_returns_not_supported_error() {
+        let cfg = TotpConfig {
+            secret: Zeroizing::new(b"12345678901234567890".to_vec()),
+            kind: TotpKind::Hotp { counter: 0 },
+            ..TotpConfig::default()
+        };
+        let result = compute_totp_code(&cfg, 0);
+        assert!(matches!(result, Err(RelicarioError::HotpNotSupported)));
     }
 
     #[test]