docs(release): add integration pilot report contract

[jeremi]

What changed

• add a bounded external integration pilot report template under the release-owned conformance packet
• link the template from the integration evidence README
• add a regression test that preserves the public evidence and privacy contract

Why

The integration exercise packet needs a narrow, reusable way to report an independently operated pilot without copying restricted evidence into the repository or overstating one run as broad production readiness.

Implementer impact

Operators get explicit fields for the validated sanitized result, frozen candidate, independence confirmation, bounded findings, accepted limitations, and conclusion. The template keeps credentials, source and operator identifiers, raw audit material, private evidence, and restricted links out of the public report.

Verification

• python3 -m unittest release/scripts/test_integration_e2_runner.py
• python3 -m unittest release/scripts/test_registry_release.py
• python3 -m unittest release/scripts/test_registry_release_plans.py
• python3 -m unittest release/scripts/test_check_release_image_oci_labels.py
• python3 -m unittest release/scripts/test_check_gates_inventory.py
• python3 -m unittest release/scripts/test_check_release_source_model.py
• release/scripts/registry-release validate release/manifests/registry-stack-beta-16.yaml
• release/scripts/registry-release validate-docsets
• REGISTRY_RELEASE_SOURCE_MODE=monorepo release/scripts/check-release-source-model.sh
• python3 release/scripts/check-debian13-images.py
• release/scripts/smoke-release-image-oci-labels.sh
• git diff --check

[jeremi]

Superseded by #469, the single ready integration PR for this reviewed 1.0 batch. This PR's exact head is preserved as an ancestor of #469, so its implementation and commit history are retained; only the merge path has been consolidated.