IdentityMapper v0.36.1

Implementer's guide documentation patch. Adds docs/implementers-guide.md with practical guidance for writing compatible implementations without reading the Python source code. Documents source provider, target identity mapper, target identity resolver, credential handling, compliance checklist, forbidden shortcuts, and minimal implementation shape. Core domain, capability contracts, runtime behavior, providers, and target mappers remain unchanged. Verified: 749 tests passed, 5 compliance tests passed, compileall OK.