Current state for N markets in one call

Fetch current price/volume for up to 50 markets in one call, so a watchlist check doesn't need N sequential tool calls. Every quote carries fetched_at and age_seconds -- the freshness signal to make your own trust call on, since both venues rate-limit hard and a quote can otherwise look fresh when it isn't. One bad ref returns an error on that entry only; the rest of the batch still succeeds.