Normalize calendar month overflow at year boundaries

[OskarEichler]

Summary

Adding 13 months to November produces an intermediate month 24; the old division maps it to month zero and raises ArgumentError. Normalize the zero-based month using divmod, including negative offsets.

Reproduction

Run after require 'fugit':

p (Fugit::Duration.parse('13M') + Time.utc(2024, 11, 1)).strftime('%Y-%m-%d')
# now "2025-12-01"; previously ArgumentError: mon out of range

Verification

• rbenv Ruby 4.0.6, TZ=UTC. Master baseline and this individual patch: 1,524 existing tests, 3,379 assertions, zero failures, one existing pending test.
• 877 focused checks for this change, run outside the repository. The combined release-based branch passes 1,517 existing RSpec examples, zero failures, one pending, plus 998 focused checks.
• Also checked the combined master changes with unmodified registry EtOrbi 1.4.1/Raabro 1.5.0: 1,524 tests/3,379 assertions, zero failures, one pending, and 997 focused checks. The extra blank-line check requires the independently fixed Raabro empty-input behavior; it is not claimed to pass on unmodified Raabro.
• Individual/release checks used the same versions of EtOrbi/Raabro with their separately proposed fixes. Syntax and whitespace checks pass. Combined source has the same 18 pre-existing Lint warnings, with no new warnings. Release gem packaging checked.

Compatibility and limitations

Previously failing positive year-boundary offsets now work. Negative offsets and the existing end-of-month overflow policy remain unchanged. Fractional-month semantics are not redefined.

No dependency/version upgrades, test additions or test modifications are included. The task's no-new-tests constraint was followed; reproduction/check scripts stayed outside the repository. Other Ruby versions and platforms await upstream CI. No production execution was performed. Prepared with Codex assistance.