Adds per-instance :zone configuration and ZonedDateTime end-to-end
coercion. Breaking change: the lower-level entities/->rate-info,
entities/->value-data, and entities/->rin-list-entry coercion fns
now require an explicit ZoneId argument; the response-helpers
(rate-info, rin-list, historical-list, historical-data) read
:midas/zone from the response automatically.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>