numfmt: fix rounding for small scaled values

[hayato0909]

Summary

Fixes rounding for small scaled values when numfmt --to=si/iec/iec-i does not select a suffix.

In this case GNU numfmt appears to use display formatting rounding for the scaled value, while uutils was applying the configured rounding method directly. This caused cases like --to=si 0.4 and --to=iec --to-unit=689 701 to round away from GNU behavior.

Changes

• Keep the existing --to=none rounding path unchanged.
• Adjust the suffix-less --to=si/iec/iec-i path to match GNU display rounding behavior.
• Add regression tests for small scaled values, --to-unit, and --format precision.

Test plan

• cargo fmt
• cargo test -p uu_numfmt
• cargo test --test tests test_numfmt::test_to_unitless_small_values_use_display_rounding
• cargo test --test tests test_numfmt::test_to_unit_with_unitless_small_value_uses_display_rounding
• cargo test --test tests test_numfmt::test_format_with_precision_and_unitless_to_arg

Fixes #11938

[github-actions]

GNU testsuite comparison:

Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Note: The gnu test tests/tail/pipe-f is now being skipped but was previously passing.

[cakebaker]

Thanks for your PR!