Add consumer migration conformance

[changeroa]

Summary

• define a closed 13-dimension consumer migration contract with traceable adoption mappings and pinned consumer-source identity
• add source-bound page-evidence start/finalize/validate lifecycle, strict artifact limits, and explicit-deadline freshness auditing
• execute governed Node unit/integration proofs plus a blocking 147-case Chromium semantic and axe matrix with 8 negative controls
• publish the migration-readiness method, Quality gate/evidence boundaries, IA routes, governance, and staged CI jobs

Verification

• consumer conformance: 51 scenarios
• page evidence: 38 scenarios, including 34 adversarial cases
• freshness: 20 scenarios
• browser matrix: 147/147 with @axe-core/playwright@4.12.1 and the declared WCAG tags
• browser/evidence sentinels: 8/8
• governance: 170/170
• component-state evidence, docs/IA/workflow/link checks, package validation, and generated-artifact checks: pass
• every one of the 8 ordered Plan-Task commits passes its scoped verification

Boundaries

• PR base is the stacked codex/roadmap-07-final-hardening branch
• playwright.config.mjs, existing baselines, snapshots, and calibration rules are unchanged
• raster capture remains advisory/nonblocking; deterministic semantic and axe checks are blocking
• no .omo/** files are committed