Range-check the integral Rational offset

[sjh9714]

Summary

Fixes #181.

offset_to_sec range-checks the resulting number of seconds in every branch
except one path through the Rational branch. When the day fraction is an
integral Rational, n is assigned inside the if arm and reaches *rof
without passing the n < -DAY_IN_SECONDS || n > DAY_IN_SECONDS guard that
sits in the else arm.

DateTime.new(2024, 1, 1, 0, 0, 0, 2).iso8601                  # => "2024-01-01T00:00:00+00:00"
DateTime.new(2024, 1, 1, 0, 0, 0, Rational(2, 1)).iso8601     # => "2024-01-01T00:00:00+48:00"
DateTime.new(2024, 1, 1, 0, 0, 0, Rational(49710, 1)).iso8601 # => "2024-01-01T00:00:00-06:28"

The Integer 2 is rejected and falls back to +00:00, but Rational(2, 1) is
the same quantity and yields a 48-hour offset. Rational(49710, 1) is
4_294_944_000 seconds, over INT_MAX, so the (int) narrowing turns a large
positive offset into a negative one.

Changes

• Move the existing range check below the if/else in offset_to_sec so it
  covers both arms. This is the same check the T_FIXNUM, T_FLOAT and
  T_STRING branches already apply, and it bounds n before the long to
  int narrowing. The rounded: label path falls through to it as well.
• Add test_civil__offset covering the three values above plus the in-range
  boundary. Rational(1, 1) is exactly DAY_IN_SECONDS and the guard is
  inclusive, so ±24:00 still round-trips.

After the change all three expressions above produce +00:00, matching the
Integer path, and the constructor emits the usual invalid offset is ignored
warning.

Testing

• bundle exec rake compile
• bundle exec rake test — 145 tests, 162600 assertions, 0 failures (144 tests
  before the new one; it fails on master and passes with the fix)
• bundle exec rake — exit 0

Run on ruby 4.0.6 (arm64-darwin25) against 83eb9d4.