Add fail-closed accessibility evidence pipeline #1638

Description

Add a fail-closed accessibility evidence pipeline that binds automated and qualified results to exact source, build, browser, fixture, and method identities. Preserve non-attestation and prevent missing, stale, or ambiguous evidence from being promoted to release-ready status.

Acceptance Criteria

• Canonical asset journeys and requirement-method mappings cover the selected accessibility scope.
• Workflow producers emit exact method cells and deterministic source manifests.
• Composition rejects missing artifacts, dirty release source, stale evidence, conflicts, and incomplete qualified review.
• Contrast review retains signature-level crops and tuple digests without manufacturing approvals.
• Reviewer handoff generation creates one privacy-minimized template per human-owned cell.
• Release and deployment workflows consume the same validation manifest and retain attestation: false.

Related

• Evidence source: comprehensive accessibility review
• Scope: repository accessibility evidence contracts and GitHub Actions