head: continue after per-file read error

[rossilor95]

Fixes #11846.

head now reports per-file read errors correctly and continues processing the remaining input files.

Previously, reading from files like /proc/self/mem could be reported as a standard output write error because io::copy mixed read and write failures into the same error path.

This change keeps read errors associated with the input file, while preserving stdout write error handling.

[oech3]

Would you avoid same letter n?

[sylvestre]

and avoid one letter variable :)

[xtqqczze]

A single letter variable is idiomatic for a variable bound by a pattern, but the outer variable should be named something like bytes_read.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/head/head-write-error. tests/head/head-write-error is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/tail/follow-name (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.

[codspeed-hq]

Merging this PR will degrade performance by 3.91%

❌ 1 regressed benchmark
✅ 316 untouched benchmarks
⏩ 46 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	ls_recursive_balanced_tree[(6, 4, 15)]	48.9 ms	50.9 ms	-3.91%

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 rossilor95:head-continue-after-read-error (f8778ad) with main (873a7c7)}

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. ↩