grep: separate context groups across files

[SichenLiang]

Track whether an earlier file printed a context group so that the first group in a later file receives a separator. The existing line-gap check still decides when two groups within the same file need one.

Tests cover the default separator, --no-group-separator, a custom separator, and recursive searches over two directories.

One case remains: if a binary file matches before a text file, GNU starts the text output with a separator, while this change does not. For example:

printf 'hit\0\n' > binary
printf 'hit\n' > text
grep -C0 hit binary text

GNU stdout begins with --\ntext:hit\n; this change prints text:hit\n. Both report the binary match on stderr and exit 0.

Fixes #130

[codspeed]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

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_{Comparing SichenLiang:context-separator-across-files (103c65a) with main (381695b)}

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. ↩