Validate natural interval steps in the correct clock fields

[OskarEichler]

Summary

Strict natural-interval validation reads the start of a slash expression instead of its step; */3 therefore becomes zero and valid schedules are rejected. It also relies on positions in the assembled cron fields, which shift when seconds/timezones are present. Validate the original hour/minute pair and read capture 2.

Reproduction

Run after require 'fugit':

p Fugit.parse_nat('every 3 hours', strict: true).to_cron_s # "0 0,3,6,9,12,15,18,21 * * *"
p Fugit.parse_nat('every 17 hours', strict: true) # nil
# previously both were nil

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.
• 20 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

Strict mode now accepts valid intervals instead of rejecting all wildcard steps. Existing hour-divisor and 1..60-minute policies are retained; this does not impose new divisibility rules on minutes or redefine seconds 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.