date: fix %N modifier handling

[dragutreis]

Fixes #11658

Fix %N (nanoseconds) strftime modifier handling to match GNU behavior:

• %_3N now correctly truncates to 3 digits and right-fills trailing zeros with spaces (e.g., "0 " instead of "0")
• %-N now correctly returns all 9 digits when no width is specified (e.g., "000000000" instead of "0")

The fix handles %N's unique semantics: width truncates from the right (not expands), _ flag right-fills with spaces, and - without width is a no-op.

Includes unit tests and integration tests.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/retry. tests/tail/retry is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/rm/isatty (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.