test(conformance): enforce Contract Truth v1

[berntpopp]

What changed

• vendor the canonical Contract Truth v1 conformance helper byte-for-byte
• add repository-specific response-envelope conformance coverage
• record the backend as verified only by its own focused and full local checks

Why

The fleet needs a central executable response-envelope contract so silent empty filters, dishonest totals or truncation, unnamed invalid parameters, and missing isError markers cannot drift independently.

Impact

This backend now gates the response behavior agents rely on against the canonical contract. No runtime, release, or fleet state is claimed beyond the evidence in this repository.

Validation

• focused conformance test passed before implementation handoff
• make ci-local passed in the isolated worktree

Part of berntpopp/genefoundry-router#68.

[berntpopp]

P0 merge gate status (live recheck 2026-07-19): head 90e6d30a9da2aeb7ffdbed016bf124dab4c5e8fb has fresh successful CI, Container scan/SBOM, repository CodeQL, conformance, and Dependency review. A separate GitHub CodeQL check (https://github.com/berntpopp/stringdb-link/runs/88223840972) has remained QUEUED since 18:25 CEST, leaving merge state UNSTABLE. This is an external GitHub gating condition; no bypass, merge, or workflow change will be used. Re-evaluate once that check completes.