MCP (Model Context Protocol) server and client for Raxol.

Provides a standalone MCP implementation with:

• Raxol.MCP.Protocol -- JSON-RPC 2.0 encoding/decoding
• Raxol.MCP.Registry -- ETS-backed tool/resource registration
• Raxol.MCP.Server -- transport-agnostic message router
• Raxol.MCP.Transport.Stdio -- stdio transport for CLI tools
• Raxol.MCP.Transport.SSE -- HTTP/SSE transport (Plug-based)
• Raxol.MCP.Client -- client for consuming external MCP servers