sort: report merge output flush errors

[DePasqualeOrg]

Unlike ordinary sort output, sort --merge did not flush its output before returning. This could hide a write failure and cause sort to exit successfully. This change adds the missing flush and a regression test.

An automated review with Codex found separate error-handling problems that are not addressed here. I'm flagging them for potential future work:

• External-sort temporary-file writes still use unwrap(), so an I/O failure can panic instead of being reported normally.
• Plain temporary files rely on drop-time flushing, which cannot report a final flush error.
• Compressed temporary-file failures require careful cleanup so that the compressor’s input is closed and the child is reaped while preserving the original I/O error. Current error paths do not always guarantee this.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/dd/no-allocate is now passing!

[codspeed-hq]

Merging this PR will regress 1 benchmark

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
❌ 1 regressed benchmark
✅ 345 untouched benchmarks
🆕 10 new benchmarks
⏩ 46 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	numfmt_large_numbers_si[10000]	94.7 ms	98.1 ms	-3.4%
⚡	Simulation	df_with_path	704.9 µs	573.7 µs	+22.86%
🆕	Memory	ptx_input_references_short_lines[100000]	N/A	222.2 MB	N/A
🆕	Memory	ptx_long_lines[100]	N/A	29.5 MB	N/A
🆕	Memory	ptx_short_lines[100000]	N/A	240.1 MB	N/A
🆕	Memory	ptx_input_references_long_lines[1000]	N/A	29.1 MB	N/A
🆕	Memory	ptx_tex[10000]	N/A	20.6 MB	N/A
🆕	Simulation	ptx_input_references_short_lines[100000]	N/A	16.2 s	N/A
🆕	Simulation	ptx_long_lines[100]	N/A	1.7 s	N/A
🆕	Simulation	ptx_short_lines[100000]	N/A	14 s	N/A
🆕	Simulation	ptx_input_references_long_lines[1000]	N/A	2 s	N/A
🆕	Simulation	ptx_tex[10000]	N/A	2.9 s	N/A

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing DePasqualeOrg:sort-output-write-errors (4265a9c) with main (66cf72b)²}

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. ↩

2. No successful run was found on main (e1efa7d) during the generation of this report, so 66cf72b was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩