Fixes #159708.

Near the last explicit TZif transition, the recurring footer can select an offset that maps a local time to a UTC instant before the footer applies. In America/Nuuk, this makes an ordinary local time use the wrong offset and change by an hour on a UTC round trip.

Check the implied UTC instant before accepting the recurring rule's result; use the final explicit time type if that instant precedes the last transition. Apply the same check in the C and Python implementations.

The regression uses generated TZif data for single-transition and multiple-transition files. It checks offsets, DST, names, and UTC round trips around the boundary with both fold values, without depending on system timezone data.

Validation on Linux x86-64 debug builds:

• The regression fails in eight subcases on unmodified main, covering both implementations, and passes with the fix.
• test_zoneinfo and test_datetime passed in GIL and free-threaded builds (1,445 test cases each, 33 skips; tzdata==2025.3 available).
• ./python -m test -R 3:3 -m test_footer_fold_at_last_transition test_zoneinfo passed with no reference leaks in both builds.
• Ruff, NEWS reStructuredText lint, patchcheck, and git diff --check passed.

• Issue: ZoneInfo.utcoffset() gives the wrong offset near the last explicit TZif transition #159708