date: fix strftime flags and widths on %N (nanoseconds)

[sjhddh]

Summary

Fixes #11658

GNU date treats %N width as precision (number of fractional-second digits) and its zeros as significant content, not padding. Previously uutils ignored or mishandled flags/widths on %N:

• %-N at @0 produced 0 instead of 000000000
• %_3N at @0 produced 0 instead of 0
• %3N at @0 produced 0 instead of 000

Changes

• Added apply_nanosecond_modifiers() in format_modifiers.rs with special handling for %N:
  • %-N: preserve all digits unchanged (zeros are content, not padding)
  • %3N: truncate to first 3 fractional digits
  • %_3N: truncate to 3 digits, replace trailing zeros with spaces
  • %_N: full 9 digits, trailing zeros replaced with spaces
• Added unit tests for nanosecond modifier handling (zero and non-zero nanos)
• Un-ignored the existing integration test test_date_strftime_n_width_and_flags

Test plan

• All 23 format_modifiers unit tests pass (including 2 new nanosecond tests)
• All 129 test_date integration tests pass (previously ignored test now runs)
• Verified: %-N at @0 → 000000000
• Verified: %_3N at @0 → 0 (0 + two spaces)
• Verified: %3N at @0 → 000

🤖 Generated with Claude Code

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/dd/no-allocate is now passing!
Congrats! The gnu test tests/printf/printf-surprise is now passing!

[cakebaker]

Can you please run cargo clippy? Thanks.

[sjhddh]

Ran cargo clippy -p uu_date — clean, no warnings:

    Checking uu_date v0.8.0 (/Users/jiahao/Git/coreutils-work/src/uu/date)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 10.92s

(The previous clippy fixes from commit ccff54f already resolved the chars_last_cmp and unnecessary_wraps lints.)