datetime: add compact timedelta formatting for zero-day durations

[14NGiestas]

Summary

This PR improves duration formatting in the datetime module by adding compact, human-friendly output for zero-day timedeltas while preserving the existing verbose format for nonzero-day values.

What changed

• Updated format_timedelta in src/datetime/stdlib_datetime.f90 to use compact zero-day formatting:
  • 5ms for sub-second durations
  • 1.500s or 42s for sub-minute durations
  • MM:SS when hours are zero
  • HH:MM:SS[.mmm] otherwise
• Preserved verbose formatting for nonzero-day durations: X days, HH:MM:SS[.mmm].
• Updated datetime spec text and examples in doc/specs/stdlib_datetime.md.
• Added example output cases in example/datetime/example_datetime_usage.f90.
• Expanded and registered unit tests in test/datetime/test_datetime.f90.

Why

The prior formatting was always verbose, which made short durations harder to scan. Compact formatting improves readability for common short intervals while preserving the legacy representation where day-level context is meaningful.

Validation

• test_datetime includes new compact-format coverage and legacy-format checks.
• Local run passed for datetime tests after preprocessing/build setup.

Compatibility

• Backward compatible for nonzero-day durations (verbose format unchanged).
• Intentional output change for zero-day durations to compact formats.

[codecov]

Codecov Report

❌ Patch coverage is 85.07463% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.88%. Comparing base (4c8521d) to head (244ffba).

Files with missing lines	Patch %	Lines
example/datetime/example_datetime_usage.f90	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1185      +/-   ##
==========================================
+ Coverage   68.81%   68.88%   +0.07%     
==========================================
  Files         408      408              
  Lines       13726    13786      +60     
  Branches     1552     1568      +16     
==========================================
+ Hits         9446     9497      +51     
- Misses       4280     4289       +9     

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