Reference data

The engine maintains three reference-data caches that hydrate the per-tick hot path with values that are too expensive to derive on the fly.

Cache	Purpose
Symbol classifier	OPRA-root → underlying-symbol normalisation; tape-class lookup.
Open-interest cache	Market-wide option OI snapshot, refreshed from flatfile_request_decoded(OpenInterest).
Daily-close cache	Prior-session close per contract; consumed by IV-change, GEX, and the 52-week band analytics.

Reference data is loaded during AnalyticsClient::connect. Loading is fail-closed: a missing cache aborts startup. The crate ships no lazy / on-demand reference-data path.