test(core): integration tests for format v2 invariants

VERSION_BYTE = 0x02; v1 blobs rejected with UnsupportedFormatVersion;
length-prefix Argon2 input distinguishes collision-engineerable pairs
(audit H1 regression test).

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

crates/idfoto-core/tests/format_v2.rs

@@ -0,0 +1,54 @@
+//! Format v2 invariants: VERSION_BYTE = 0x02, v1 blobs are rejected with
+//! UnsupportedFormatVersion, length-prefix construction guarantees domain
+//! separation.
+
+use idfoto_core::{
+    IdfotoError,
+    crypto::{KdfParams, VERSION_BYTE},
+    decrypt, derive_master_key, encrypt,
+};
+use zeroize::Zeroizing;
+
+fn fast_params() -> KdfParams { KdfParams { argon2_m: 256, argon2_t: 1, argon2_p: 1 } }
+
+#[test]
+fn version_byte_is_2() {
+    assert_eq!(VERSION_BYTE, 0x02);
+}
+
+#[test]
+fn fresh_ciphertext_starts_with_0x02() {
+    let key = Zeroizing::new([0u8; 32]);
+    // encrypt(key: &[u8; 32], plaintext: &[u8])
+    let ct = encrypt(&*key, b"hello").unwrap();
+    assert_eq!(ct[0], 0x02);
+}
+
+#[test]
+fn v1_blob_is_rejected_with_unsupported_format_version() {
+    // v1 layout: [0x01][24 nonce bytes][16 tag bytes]
+    let mut blob = vec![0x01u8];
+    blob.extend_from_slice(&[0u8; 24 + 16]);
+    let key = Zeroizing::new([0u8; 32]);
+    // decrypt(key: &[u8; 32], data: &[u8])
+    let err = decrypt(&*key, &blob);
+    match err {
+        Err(IdfotoError::UnsupportedFormatVersion { found, expected }) => {
+            assert_eq!(found, 0x01);
+            assert_eq!(expected, 0x02);
+        }
+        other => panic!("expected UnsupportedFormatVersion, got {other:?}"),
+    }
+}
+
+#[test]
+fn length_prefix_distinguishes_concat_collisions() {
+    let salt = [0u8; 32];
+    let img = [0x44u8; 32];
+    let p1 = b"abc";
+    let p2 = b"abcD";  // Pre-length-prefix, ("abc", [0x44, ...]) and ("abcD", ...)
+                       // could be made to collide. With length-prefix they cannot.
+    let k1 = derive_master_key(p1, &img, &salt, &fast_params()).unwrap();
+    let k2 = derive_master_key(p2, &img, &salt, &fast_params()).unwrap();
+    assert_ne!(*k1, *k2);
+}