MCP E-Invoicing Core

This is a foundation library, not a standalone server. It provides abstract base classes, shared Pydantic models, and reusable utilities for building country-specific e-invoicing MCP servers across Europe. You get `BaseDocumentGenerator` for XML creation, `BaseLifecycleManager` for OAuth2-backed platform APIs, and `BasePartyValidator` for tax ID checks. The included `InvoiceDocument` model follows EN 16931 to future-proof against the 2030 ViDA mandate. France's `mcp-facture-electronique-fr` uses the OAuth client for Chorus Pro, Italy's `mcp-fattura-elettronica-it` uses the XML utils for FatturaPA generation, and Belgium, Germany, Poland, and Spain adapters inherit from the same base. Install this if you're building a new country adapter or need to understand how the existing French and Italian servers share code.