uniq: optimize memory usage for ignore-case comparison

[CrazyRoka]

Previously, when running uniq -i, the implementation eagerly allocated a new vector and copied the lowercased line into it for every single line read. Profiling indicated that build_meta accounted for a significant portion of execution time (approx 27%) due to this allocation.

This change removes the lowercase buffer from LineMeta entirely. Instead, it implements a lazy, iterator-based comparison in keys_differ. This eliminates the per-line allocation overhead and enables short-circuiting during comparison.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/printf/printf-surprise is now passing!

[codspeed-hq]

CodSpeed Performance Report

Merging #10050 will improve performance by 7.04%

_{Comparing CrazyRoka:uniq-optimize-case-sensitive (28deca2) with main (666c6df)}

Summary

⚡ 1 improvement
✅ 138 untouched
⏩ 37 skipped¹

Benchmarks breakdown

	Benchmark	BASE	HEAD	Efficiency
⚡	uniq_case_insensitive[10000]	1.8 ms	1.6 ms	+7.04%

Footnotes

1. 37 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. ↩

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tty/tty-eof. tests/tty/tty-eof is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[cakebaker]

Nice refactoring, thanks!