MercuryMesh MVP

- Lightweight, contract-driven primitives for privacy-preserving cross-venue analytics.
- Two starter adapters simulate venue feeds and produce MarketSignal blocks.
- Includes a simple in-tree Graph-of-Contracts registry and data models.

Usage:
- Import mercurymesh.contracts to construct MarketSignal and AggregatedSignal.
- Use mercurymesh.registry.GraphOfContractsRegistry to register adapters and contracts.