What would you like to be added:
I’d like to contribute a new test file that adds comprehensive unit and benchmark tests for the hash functions used in the project. This would cover key cases for Hash128, rotl64, and fmix64, while also ensuring determinism and robustness for a variety of inputs.

Why is this needed:
Currently, there is limited unit test coverage for the hash package. Adding these tests will help improve code reliability, simplify future refactoring, and provide useful benchmarks for performance analysis. This should also help other contributors feel more confident when working on hashing logic in the codebase.