fuzz: add differential fuzzer against GNU grep

[sylvestre]

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).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.28%. Comparing base (c614a57) to head (9598faf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main       #7   +/-   ##
=======================================
  Coverage   95.28%   95.28%           
=======================================
  Files           6        6           
  Lines        1422     1422           
  Branches      140      140           
=======================================
  Hits         1355     1355           
  Misses         66       66           
  Partials        1        1           

Flag	Coverage Δ
macOS_latest	96.09% <ø> (ø)
ubuntu_latest	96.09% <ø> (ø)
windows_latest	0.00% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[lhecker]

I believe most of this is adopted from the coreutils repo, right? I saw similar code there as well.

Personally speaking, I think some of the text feels somewhat verbose. And as a Windows Terminal maintainer, I also always get a bit squirmish when I see crates like console being used. Their Windows implementation is usually not great and somewhat bodgy.

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

---

_{Comparing add-differential-fuzzer (9598faf) with main (c614a57)}

Footnotes

1. 17 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩