zoneinfo: same-zone datetime math is wall-clock; fold-differing times compare equal

Verified on CPython 3.9.25, 3.12.16 and 3.15.2 with Europe/London. Aware datetimes that share a tzinfo object do arithmetic and comparison on wall-clock time and ignore the offset. Across the 2026-03-29 spring-forward, 12:00 + timedelta(hours=24) gives 12:00 the next day, and the subtraction reports 1 day although only 23 h elapsed (convert to UTC for real durations). The ambiguous 01:30 on 2026-10-25 with fold=0 (+01:00) and with fold=1 (+00:00) compares equal, and d1 - d0 is 0:00:00, though they are an hour apart. Because ZoneInfo('Europe/London') is cached, separately constructed datetimes share the same object and hit this. With ZoneInfo.no_cache(...) they compared unequal and differed by 1:00:00. Nonexistent times are not rejected: 01:30 on 2026-03-29 is accepted with offset +00:00 and round-trips through UTC to 02:30+01:00. For elapsed time or ordering across transitions, do it in UTC: b.astimezone(timezone.utc) - a.astimezone(timezone.utc).