Add a cargo-fuzz harness (fuzz_grep) that runs uu_grep and GNU grep on
the same generated args/input and panics on any divergence, using the
vendored uufuzz crate (adapted from uutils/coreutils to depend on
crates.io uucore rather than a path).
A CI workflow (.github/workflows/fuzzing.yml) builds the uufuzz
examples, builds the fuzzer, and runs it for 60s. fuzz_grep is marked
should_pass: false / continue-on-error since it currently surfaces real
GNU-compatibility gaps (e.g. uu_grep rejects a repeated -m, GNU accepts).
Sylvestre Ledru