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/common/mod.rs

@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: MIT-0
+//
+// Fixtures shared by the integration-test crates (tests/*.rs). The unit
+// tests inside src/ cannot reach this module and keep their own copies.
+
+// Each test crate compiles its own copy of this module and not every crate
+// uses every fixture, so the dead-code lint misfires here.
+#![allow(dead_code)]
+
+use fcry::SecretBytes32;
+
+/// The raw 32-byte key used by all integration tests.
+pub const KEY: &[u8; 32] = b"0123456789abcdef0123456789abcdef";
+
+/// [`KEY`] wrapped in the `SecretBytes32` the library API takes.
+pub fn test_key() -> SecretBytes32 {
+    let mut key = SecretBytes32::zeroed();
+    key.with_mut_array(|key| key.copy_from_slice(KEY));
+    key
+}

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() {

tests/roundtrip.rs

@@ -15,7 +15,8 @@ use std::{
 use assert_cmd::cargo::CommandCargoExt;
 use tempfile::TempDir;
 
-const KEY: &[u8; 32] = b"0123456789abcdef0123456789abcdef";
+mod common;
+use common::KEY;
 
 fn fcry() -> Command {
     Command::cargo_bin("fcry").unwrap()