grep: reject conflicting matcher flags

[wondr-wclabs]

Closes #3.

GNU grep treats matcher selection as a choice between modes: -F, -E, -G, and -P can each be repeated idempotently, but mixing different modes is a usage error. The old parser model used Clap overrides, which made mixed modes silently become “last flag wins.”

This changes the parser to preserve which matcher flags were present and then performs a small GNU-style validation step in uumain. I used custom validation instead of Clap conflicts so the diagnostic matches the behavior described in the issue (conflicting matchers specified) while still accepting repeated identical flags such as -F -F and -E -E.

The regression test covers the listed mixed-mode combinations, a long-option pair, and repeated same-mode controls.

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

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_{Comparing wondr-wclabs:codex/reject-conflicting-matchers (2a9296c) with main (d28bf76)}

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. ↩