External implementation experiment checkpoint. Adds docs/external-implementation-experiment.md, defining a protocol documentation experiment for implementing a minimal FakeToken Source Provider, Demo Target Identity Mapper, and Demo Target Identity Resolver without reading the Python reference source code. Defines allowed documents, forbidden source/test access, required flow, required tests, forbidden shortcuts, expected report, and success/fail criteria. Core domain, capability contracts, runtime behavior, providers, and target mappers remain unchanged. Verified: 749 tests passed, 5 compliance tests passed, compileall OK.