Convert a datetime between any two IANA time zones using the runtime's ICU tzdb — always current, DST-correct, half-hour zones included. A datetime without offset is read as wall time in `from` (ambiguous fall-back times resolve to the earlier instant; times skipped by spring-forward shift ahead, flagged in `note`). Returns both ISO datetimes with offsets and DST status per side. Unknown zone → 400 with a spelling suggestion.