chore: prepare v3.1.0 release proof

[aatuh]

Summary

• refresh .next_steps.md as the v3.1.0 release-and-proof checklist
• add checked-in reference saas-api-full service evidence and docs
• record validation adapter drift disposition and refresh v3.1 release evidence

Checks

• GOWORK=off GOTOOLCHAIN=local make reference-service-check
• GOWORK=off GOTOOLCHAIN=local make docs-check
• GOWORK=off GOTOOLCHAIN=local make coverage-check
• GOWORK=off GOTOOLCHAIN=local make finalize
• API_BASE_REF=v3.0.2 GOWORK=off GOTOOLCHAIN=local make release-check
• API_BASE_REF=v3.0.2 GOWORK=off GOTOOLCHAIN=local make release-evidence
• RELEASE_SUMMARY=release-check-summary.json make release-review-summary
• GOWORK=off GOTOOLCHAIN=local make github-governance-check

Integration Evidence

• Local Docker integration was not used as publication evidence because this shell cannot reach /var/run/docker.sock.
• Manual generated-full-integration workflow runs will be triggered on this branch for include-minio=false and include-minio=true.

[aatuh]

Release proof update for v3.1.0:

• Local governance verifier passed: GOWORK=off GOTOOLCHAIN=local make github-governance-check.
• Local release check passed: API_BASE_REF=v3.0.2 GOWORK=off GOTOOLCHAIN=local make release-check.
• Local release evidence passed and release-check-summary.json is publication-eligible.
• Reviewer summary passed: RELEASE_SUMMARY=release-check-summary.json make release-review-summary.
• Generated full integration without MinIO passed: https://github.com/aatuh/api-toolkit/actions/runs/26147744213
• Generated full integration with MinIO passed: https://github.com/aatuh/api-toolkit/actions/runs/26147744208

Remaining blocker before tag/publish: CODEOWNERS review is required by branch protection. After approval/merge, create v3.1.0 from master, let the release workflow regenerate tag evidence, download draft assets to .ci-result/release-assets/v3.1.0, and run RELEASE_ASSET_DIR=.ci-result/release-assets/v3.1.0 RELEASE_ARTIFACT_VERIFY_MODE=publication RELEASE_TAG=v3.1.0 GITHUB_REPOSITORY=aatuh/api-toolkit make release-artifact-verify.

[aatuh]

Owner release approval recorded in this thread. GitHub cannot count the sole author's self-review as CODEOWNERS approval, so this PR is being merged with admin privileges after all required checks and generated integration evidence passed. Head commit being merged: 95dd985.