Add mutation testing to verify test quality

[Amayyas]

What

30 passing specs doesn't guarantee they'd actually catch a regression. Mutation testing (e.g. the mutant gem, or a lighter alternative if mutant's licensing/setup is too heavy for this project's size) deliberately introduces small code mutations and checks whether the test suite fails.

Acceptance criteria

• Evaluate tooling options (mutant vs alternatives) and pick one
• Wire into CI (can be non-blocking / informational at first)
• Document how to run it locally