stat: truncate string precision on a byte boundary to avoid a panic

[leeewee]

Fixes #13750

stat's print_str applied a %.P precision by slicing the &str value
(&s[..p]). When the precision P lands inside a multibyte UTF-8 character —
e.g. stat -c '%.9n' on a file whose name ends in a two-byte character — the
slice is not on a char boundary and stat panics and aborts (exit 134). GNU
stat truncates by bytes and exits 0.

$ : > "$(printf 'é')" && stat -c '%.1n' "$(printf 'é')"
thread 'main' panicked at src/uu/stat/src/stat.rs:409:50:
end byte index 1 is not a char boundary; it is inside 'é' (bytes 0..2 of string)

Fix

Route print_str through the existing write_padded_bytes helper — the same one
print_os_str already uses on Unix — so truncation and padding operate on bytes.
The output now matches GNU stat byte-for-byte (the partial byte is emitted, as
GNU does) instead of crashing.

Test

Adds test_precision_splits_multibyte_char_in_value, which asserts that a
precision splitting a multibyte character truncates on the byte and succeeds.

[codspeed-hq]

Merging this PR will degrade performance by 18.29%

⚠️ 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 regressed benchmark
✅ 346 untouched benchmarks
⏩ 46 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	df_with_path	571.4 µs	699.3 µs	-18.29%

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 leeewee:stat-fix-precision-char-boundary (0cb64bb) with main (21d4e96)}

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. ↩