Call when an agent needs a near-real-time FX bid, ask, and mid for one pair such as EURUSD. Kraken's public book supplies bid and ask, which can be wide, plus the spread timestamp. If Kraken fails, Yahoo's last price is the mid and bid and ask are null. Distinct from daily ECB fx-rates. Send JSON {"pair":"EURUSD"}. Exact $0.003 USDC on Base. Try unpaid POST /v1/sandbox/fx-tick first.