Preserve fractional seconds in numeric duration arithmetic

[OskarEichler]

Summary

Duration#+ and #- truncate numeric operands with to_i. For example, Fugit::Duration.parse('1s') + 0.5 reports 1 second instead of 1.5. Add the numeric value without truncation.

Reproduction

Run after require 'fugit':

d = Fugit::Duration.parse('1s')
p [(d + 0.5).to_sec, (d - 0.5).to_sec] # now [1.5, 0.5], previously [1, 1]

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.
• 6 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

Fractional numeric operands now contribute their fractional seconds. Integer behavior and receiver immutability are preserved.

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.