[Core] Implement MockExchange for offline development

[realfishsam]

Purpose / Goal

Allow developers to build applications using PMXT without needing real API keys or internet access. This is essential for CI/CD environments and rapid UI prototyping.

Tasks

• Create core/src/exchanges/mock/index.ts.
• Implement the BaseExchange interface.
• Provide 'realistic but fake' data for fetchMarkets, fetchOrderBook, and fetchBalance.
• Simulate order execution (e.g., createOrder always returns a 'success' after 100ms).

Easy Way to Get Started

1. Look at core/src/BaseExchange.ts to see the required methods.
2. Copy the structure of core/src/exchanges/polymarket/index.ts but replace all HTTP calls with Promise.resolve() and local JSON data.