Add MEXC futures support (no KYC, up to 200x leverage)

- MexcFuturesClient with full API support
- Positions, orders, markets, funding rates
- TP/SL order support
- Auto-setup via MEXC_API_KEY and MEXC_API_SECRET env vars
- No KYC required for smaller trading amounts

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: alsk1992