fix: verify base rendering across RDF term positions

[jdsika]

Summary

• Verify direct and literal-datatype IRI terms in serialized RDF.
• Retain usable equal-base Turtle namespace bindings.
• Retry a failed optional base rendering without the base, retaining namespaces and requiring final verification.
• Cover query and fragment bases, typed RDF/XML literals, parser fidelity, unchanged caller state, and bounded error handling.

Verification

• Source suite: 842 passed; coverage 95.89%.
• Focused suite: 114 passed at each dependency endpoint.
• Ruff and mypy passed.
• Independent adversarial review approved; 124 ordinary outputs remained byte-identical.
• Before/after probes demonstrate preservation of query-relative and RDF/XML datatype IRIs that the base implementation changed.