fix(date): keep POSIX '.' in custom --date fractional seconds

[pnavais]

Summary

After #820, custom --date / TIME_STYLE=+… formats went through format_localized(). Chrono then uses the locale decimal separator for %.Nf, so locales like es_ES, de_DE, and fr_FR emit , instead of ..

That breaks:

• User-facing custom formats that expect POSIX strftime punctuation (e.g. +%Y-%m-%d %H:%M:%.3f)
• tests/integration.rs::test_date_custom_format_supports_nanos_with_length, which matches testDateFormat\.[0-9]{3}

DateFlag::Locale still uses format_localized() for intentional localization. Custom formats use non-localized format() again.

This may also explain failures reported in #1223 on macOS when the system locale is non-English (via sys-locale), independent of APFS timestamp precision — that test only asserts the %.3f pattern appears twice; it does not require distinct nanosecond values.

Verification

en_US: format_localized("%.3f") => .NNN
es_ES: format_localized("%.3f") => ,NNN
plain format("%.3f")            => .NNN  (all locales)

• cargo test custom_format — unit + integration tests pass
• cargo fmt

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TODO

• Use cargo fmt
• Add necessary tests
• Update README (if applicable)
• Update config sample file in doc/samples (if applicable)
• Update icon sample file in doc/samples (if applicable)
• Update color sample file in doc/samples (if applicable)
• Update man page at lsd/doc/lsd.md (if applicable)