grep: honor GNU buffer anchors

[wondr-wclabs]

Fixes #33.

GNU grep recognizes \` and \' as start/end buffer anchors in both BRE and ERE mode. In this implementation, grep searches one record/line at a time, so those GNU buffer anchors behave like the record-local start/end anchors for the cases in this issue. The current Syntax::grep() and Syntax::gnu_regex() setup did not enable Oniguruma's GNU buffer-anchor operator, so the escapes were treated as literal backtick/apostrophe characters.

This enables SYNTAX_OPERATOR_ESC_GNU_BUF_ANCHOR only for RegexMode::Basic and RegexMode::Extended. I left Fixed untouched because escapes are literals there, and left Perl untouched because -P should follow the PCRE-style syntax path rather than GNU BRE/ERE extensions.

Validation:

• cargo fmt --all -- --check
• cargo test
• cargo clippy --all-targets --workspace -puu_grep -- -D warnings
• git diff --check
• printf 'cat\ndog\n' | cargo run --quiet -- -e "t\\'" now prints cat
• printf 'cat\ndog\n' | cargo run --quiet -- -e '\c'now printscat`

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

---

_{Comparing wondr-wclabs:codex/gnu-buffer-anchors (0ec43c8) with main (f2f86ef)}

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. ↩

[lhecker]

This seems like it could be expressed in just two tests, no?

[wondr-wclabs]

Agreed. I collapsed this to two command invocations: one BRE case and one ERE case. Each pattern now covers both GNU buffer anchors in one command:

BRE: \`c\|r\'
ERE: \`c|r\'

That keeps the mode distinction explicit while avoiding four near-identical command setups. Focused validation after the change: cargo test --test test_grep gnu_buffer_anchors.

[wondr-wclabs]

I also rebased this branch onto current upstream/main after the earlier CI run. The previous branch was still stacked on already-merged grep work, so the PR diff and CI were carrying unrelated binary-path changes.

After the rebase, this PR is back to the intended scope: enabling Oniguruma's GNU buffer-anchor operator for BRE/ERE plus the two compact anchor tests. Local validation on the cleaned branch:

cargo fmt --all -- --check
cargo test --verbose

[lhecker]

Thanks, anthropic/claude-sonnet-4-6! 🥲