shred: handle '-' as stdout

[gabrielhnf]

Fixes #12288

Resolve - to /dev/fd/1 on Unix systems, allowing shred to operate on whatever stdout is connected to. This matches the UNIX convention used by most coreutils tools.

When stdout is a pipe or terminal, the existing file type validation naturally rejects it with "not a file".
This doesn't match GNU's "invalid file type", but feels out of scope for the issue.

[codspeed-hq]

Merging this PR will improve performance by 13.72%

⚠️ 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

⚡ 2 improved benchmarks
✅ 315 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Memory	cp_recursive_deep_tree[(120, 4)]	699.2 KB	562.5 KB	+24.31%
⚡	Simulation	ls_recursive_balanced_tree[(6, 4, 15)]	50.9 ms	48.9 ms	+4.03%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing gabrielhnf:fix-shred-stdout-dash (bbb9c48) with main (d09f351)}

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. ↩

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/retry (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Congrats! The gnu test tests/pr/bounded-memory is now passing!

[xtqqczze]

this comment should move down, or be removed

[gabrielhnf]

Sure!