Fix naturaltime comparisons across daylight-saving transitions - #425

Changes proposed in this pull request:

• Compare timezone-aware naturaltime() inputs as UTC instants. The current conversion to naive local time erases the repeated DST hour: in America/New_York, 2026-11-01 01:30-04:00 relative to 2026-11-01 01:30-05:00 incorrectly produces now instead of an hour ago. The spring transition can similarly report two elapsed hours for one.
• Preserve the existing local interpretation of naive inputs when mixed with aware inputs. Keep the all-naive comparison path unchanged, and use the current UTC instant when an aware input omits when.
• Add six regression cases for fallback in both directions, spring-forward, mixed naive/aware inputs, and the default current time. The initial four explicit-time cases fail before the fix.

Validation on Python 3.12.14/macOS:

• Full suite with benchmarks disabled: 750 passed, 112 skipped. The skipped translation tests require compiled gettext .mo files unavailable locally.
• Source doctests: 15 passed.
• Pinned mypy: clean across 14 source files.
• All applicable configured prek hooks passed with Python 3.12 overrides in a temporary local configuration; the unmodified configuration selected the host Python 3.9, incompatible with current Black/tox-ini-fmt. No lint configuration changes are included.
• git diff --check passed.

Prepared and independently reviewed with OpenAI Codex assistance.