Retain fractional seconds when adding calendar durations

[OskarEichler]

Summary

Calendar year/month arithmetic reconstructs a time using integer seconds, silently dropping the input fraction and any fractional seconds already added by the duration. Preserve that remainder after reconstruction to avoid an extra formatted-time precision loss.

Reproduction

Run after require 'fugit':

p (Fugit::Duration.parse('1M0.25s') + Time.utc(2024, 1, 10, 8, 30, 12.5)).to_f % 1
# now 0.75, previously 0.0

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.
• 39 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

Year/month arithmetic now preserves fractional seconds. This retains EtOrbi's existing floating-point precision; it does not promise arbitrary precision or change end-of-month/DST policy.

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.