Timezone convert / IANA tz conversion / DST-aware time arithmetic / wall-clock to UTC / convert across cities. Convert a datetime from one IANA timezone to another. ISO-8601 input — bare wall-time strings are interpreted in the from_tz, fully-qualified offsets are honored. DST is handled correctly (uses V8's built-in tzdata via Intl.DateTimeFormat). Returns both formatted strings, both UTC offsets, and the difference in hours.