fix(wasi): sort ordering check and external-merge separator

[eduardomourar]

sort's ordering check runs single-threaded on WASI, which has no thread support, reading every chunk up front instead of streaming from a background reader thread. Its external-merge path also inserts a missing separator between concatenated files so lines don't merge across file boundaries.

[codspeed-hq]

Merging this PR will not alter performance

✅ 345 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing eduardomourar:wasi-sort-fixes (62496f4) with main (0c8a3c7)}

Footnotes

1. 46 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. ↩