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).
