cleanup: share the integration-test key fixture in tests/common

The 32-byte test key "0123456789abcdef0123456789abcdef" was hardcoded
in three places: src/crypto.rs unit tests, tests/roundtrip.rs, and
tests/library_api.rs - three copies to keep in sync if the fixture
ever changes.

Add tests/common/mod.rs exposing the KEY bytes and a test_key()
SecretBytes32 constructor; roundtrip.rs and library_api.rs now pull
from it. The unit tests in src/crypto.rs cannot reach an
integration-test module and keep their own copy.

The module carries #![allow(dead_code)] because each test crate
compiles its own copy and none uses every fixture.

Test-only change.

Test plan: cargo test passes all suites (43 CLI roundtrip tests,
2 library_api tests, 13 unit tests).

tests/library_api.rs

@@ -6,7 +6,6 @@ use fcry::{
     EncryptOptions,
     KdfParams,
     OutputOptions,
-    SecretBytes32,
     decrypt,
     decrypt_range,
     default_argon_decrypt_cap_mib,
@@ -14,11 +13,8 @@ use fcry::{
 };
 use tempfile::TempDir;
 
-fn test_key() -> SecretBytes32 {
-    let mut key = SecretBytes32::zeroed();
-    key.with_mut_array(|key| key.copy_from_slice(b"0123456789abcdef0123456789abcdef"));
-    key
-}
+mod common;
+use common::test_key;
 
 #[test]
 fn library_file_roundtrip_raw_key() {