refactor: verify backend prefix rendering without token rewrites

[jdsika]

Summary

• Delegate prefixed-name grammar to the serializer and readers; remove local token rewriting.
• Select optional base and prefix rendering through bounded, fully verified attempts, preserving ordinary compact output and unrelated backend errors.
• Separate RDF fidelity matrices from deterministic retry-policy tests, including terminal failure, successful first attempts and per-call option isolation.
• Document the conditional base/prefix presentation contract and changed output behavior.

Verification

• Ruff and mypy pass.
• Full source suite: 943 passed, 95.55% statement coverage.
• Focused base/prefix suite: 143 passed on both minimum and newest dependencies.
• Fresh wheel built from sdist; Twine checks pass for both distributions.
• Shared installed-wheel gate: 941 passed, two source-only skips; exact artifact digest and package provenance verified despite hostile import/test environment settings.
• Installed-package end-to-end workflows and eight runnable documentation examples pass; six signature blocks are parsed by the committed documentation tests.
• Scoped standards integration: 60 positive default-hash RDFC/WL vectors, 52 graph cases and 364 serializer checks. This is not a claim of full official textual conformance.
• Independent adversarial review passed: 972 ordinary-output byte comparisons, 166 state/error-boundary probes, and all 143 focused tests against simulated improved backend acceptance.