grep: reject multiple patterns with -P

[wondr-wclabs]

Fixes #34.

This brings -P closer to GNU grep for the specific PCRE restriction described in the issue: PCRE mode accepts exactly one logical pattern. Other matcher modes still keep the existing multi-pattern behavior.

The validation is placed after grep has already normalized all pattern sources into patterns because that is the point where the rule is easiest to express correctly. That means the same check covers:

• repeated -e, e.g. -P -e a -e b
• a single -e containing a newline, e.g. -P -e 'a\nb'
• a positional pattern containing a newline
• -f files with more than one pattern line

I kept this out of Clap argument validation because -e, -f, and positional patterns all flow through different input paths and only become equivalent after the existing pattern-splitting logic. Checking the normalized pattern count avoids duplicating that logic and keeps the restriction specific to RegexMode::Perl.

Validation:

• printf 'abc\n' | cargo run --quiet -- -P -e a -e b now exits 2 with grep: the -P option only supports a single pattern
• cargo test pcre_rejects_multiple_patterns -- --nocapture
• cargo test
• cargo test --no-fail-fast
• cargo fmt --all -- --check
• cargo clippy --all-targets --workspace -p uu_grep -- -D warnings
• git diff --check

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

---

_{Comparing wondr-wclabs:codex/pcre-single-pattern (7449b75) with main (f4798cb)}

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. ↩

[sylvestre]

already merged here #40