Missing tooling to run fuzzing locally

[ThetaSinner]

The flake does not provide the Cargo tool test-fuzz. It can be installed with cargo install cargo-test-fuzz but ideally we'd just have all the tools we need without doing global installs.

[ThetaSinner]

Oh you also need cargo install cargo-afl for the fuzz tests to run

[ThetaSinner]

We might prefer to focus on #115 but if we want to be able to run fuzzing locally to check that it's working or because we don't have a remote machine to test on yet, then this issue should be about making that easier. Any tools we can add through Nix, let's do that. Then any system setup that's needed beyond that should be documented.