rust-testing

Applies CipherStash conventions for Rust tests — running unit and integration tests with nextest and its test groups, nested given_* scenario modules with expectation-style names, assertion messages, randomised test data with fake and Dummy impls behind a test-utils feature, per-test mocktail HTTP mocks, paused-time tokio tests, concurrency stress tests, and testing napi-rs boundaries. Use when writing or changing Rust tests (#[test],