fix: csplit --suffix-format huge field width panics instead of erroring

[mvanhorn]

Summary

csplit no longer panics when --suffix-format requests an enormous field width. A format like --suffix-format=%0999999999d now returns a clean error instead of aborting with a Rust panic, because the split-name formatter propagates the formatting error rather than unwrapping it.

Why this matters

Issue #12763 reports that csplit panics on a --suffix-format with a very large field width. SplitName::get used write!/format! and unwrapped the result; for a pathological width the formatter allocation/formatting failed and the unwrap turned that into a panic, crashing the utility instead of reporting a usage error the way GNU csplit does.

The formatter now returns io::Result<String> and the caller surfaces a proper csplit error, so an unusable suffix format is rejected with a message and a non-zero exit rather than a panic.

Testing

cargo test -p uu_csplit passes, including a new test_csplit case asserting that a huge --suffix-format width errors out (non-zero exit, no panic) instead of crashing. cargo clippy -p uu_csplit is clean.

Fixes #12763

[codspeed-hq]

Merging this PR will improve performance by 4.88%

⚡ 1 improved benchmark
✅ 330 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	sort_ascii_utf8_locale	16.1 ms	15.4 ms	+4.88%

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.

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_{Comparing mvanhorn:fix/12763-csplit-suffix-format-width-panic (d65969e) with main (df0701c)}

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:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/misc/tty-eof (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/follow-name (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.

[sylvestre]

sorry but could you please rebase it ? thanks