grep: honor GNU buffer anchors in BRE

[wondr-wclabs]

Fixes #33.

This enables Oniguruma's GNU buffer-anchor syntax for BRE mode so escaped backtick and escaped apostrophe are parsed as start/end anchors instead of literal characters. The change is limited to BRE because the reported mismatch is in default grep mode, and the operator is the specific Onig flag for these GNU anchor spellings.

Regression coverage adds default-mode cases for both anchors, including non-matches where the same characters appear away from the line boundary.

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

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_{Comparing wondr-wclabs:codex/bre-buffer-anchors (101996d) 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. ↩

[wondr-wclabs]

Closing this older duplicate in favor of #56. The replacement PR enables the same GNU buffer-anchor operator, but covers both BRE and ERE because the repro also fails under -E; #38 only covered the default BRE path.