Function normalize_spot_symbol 

pub fn normalize_spot_symbol(symbol: &str) -> String

Normalizes a Kraken spot symbol to use BTC instead of XBT.

Kraken’s REST API returns XBT for Bitcoin (following ISO 4217 conventions), but their WebSocket v2 API uses the more common BTC format. This function normalizes symbols so that instruments and subscriptions use consistent, industry-standard symbols. Handles XBT in both base position (XBT/USD -> BTC/USD) and quote position (ETH/XBT -> ETH/BTC).