Handle scalar hours when combining natural intervals

[OskarEichler]

Summary

Combining a minute interval with a specific numeric hour probes that hour for range syntax using Integer#match, which does not exist. Convert the range candidate to text before matching, leaving the original scalar/array values unchanged.

Reproduction

Run after require 'fugit':

p Fugit.parse_nat('every 10 minutes at 5').to_cron_s
# "0,10,20,30,40,50 5 * * *"; previously NoMethodError
p Fugit.parse_nat('every 10 minutes from 5 to 8').to_cron_s
# unchanged "0,10,20,30,40,50 5,6,7 * * *"

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.
• 4 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

Valid interval-plus-hour expressions now parse instead of raising. Existing hour-range behavior is preserved. This does not redefine how separate hours combine into multiple crons; multi:true can still return separate schedules.

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.