feat(core): bump VERSION_BYTE to 0x02 with typed UnsupportedFormatVersion

Clean break from v1 — no migration. Decrypting a v1 blob now returns
IdfotoError::UnsupportedFormatVersion { found: 0x01, expected: 0x02 }.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/idfoto-core/src/crypto.rs

@@ -22,7 +22,7 @@
 //! [version: 1 byte] [nonce: 24 bytes] [ciphertext + Poly1305 tag: variable]
 //! ```
 //!
-//! - **Version byte** (`0x01`): allows future format changes without ambiguity.
+//! - **Version byte** (`0x02`): allows future format changes without ambiguity.
 //!   Decryption rejects any version it does not recognize.
 //! - **Nonce** (24 bytes): randomly generated per encryption via [`OsRng`].
 //!   Stored alongside the ciphertext so the decryptor does not need out-of-band
@@ -56,7 +56,7 @@ use zeroize::Zeroizing;
 use crate::error::{IdfotoError, Result};
 
 /// Current binary format version. Increment this if the ciphertext layout changes.
-const VERSION_BYTE: u8 = 0x01;
+pub const VERSION_BYTE: u8 = 0x02;
 
 /// XChaCha20-Poly1305 nonce length: 192 bits = 24 bytes.
 const NONCE_LEN: usize = 24;
@@ -123,12 +123,12 @@ pub fn decrypt(key: &[u8; 32], data: &[u8]) -> Result<Vec<u8>> {
         ));
     }
 
-    let version = data[0];
-    if version != VERSION_BYTE {
-        return Err(IdfotoError::Format(format!(
-            "unknown version byte: 0x{:02x}",
-            version
-        )));
+    let found = data[0];
+    if found != VERSION_BYTE {
+        return Err(IdfotoError::UnsupportedFormatVersion {
+            found,
+            expected: VERSION_BYTE,
+        });
     }
 
     let nonce = XNonce::from_slice(&data[1..1 + NONCE_LEN]);
@@ -344,8 +344,8 @@ mod tests {
         let expected_len = 1 + 24 + plaintext.len() + 16;
         assert_eq!(ciphertext.len(), expected_len);
 
-        // Version byte must be 0x01
-        assert_eq!(ciphertext[0], 0x01);
+        // Version byte must be 0x02
+        assert_eq!(ciphertext[0], 0x02);
     }
 
     #[test]
@@ -394,4 +394,27 @@ mod tests {
         let key: zeroize::Zeroizing<[u8; 32]> = derive_master_key(b"x", &img, &salt, &params).unwrap();
         assert_eq!(key.len(), 32);
     }
+
+    #[test]
+    fn version_byte_is_0x02() {
+        assert_eq!(VERSION_BYTE, 0x02);
+    }
+
+    #[test]
+    fn decrypt_rejects_v1_blob_with_typed_error() {
+        // Construct a v1-style blob: [0x01][24 nonce bytes][16 tag bytes].
+        let mut blob = vec![0x01u8];
+        blob.extend_from_slice(&[0u8; 24]);
+        blob.extend_from_slice(&[0u8; 16]);
+
+        let key = Zeroizing::new([0u8; 32]);
+        let err = decrypt(&*key, &blob).expect_err("v1 blob should fail decrypt");
+        match err {
+            IdfotoError::UnsupportedFormatVersion { found, expected } => {
+                assert_eq!(found, 0x01);
+                assert_eq!(expected, 0x02);
+            }
+            other => panic!("expected UnsupportedFormatVersion, got {:?}", other),
+        }
+    }
 }