Add structural visual evidence for frontend PRs

[bigboateng]

What changed

• add opt-in workflow.pr_visual_evidence policy with off, suggest, and require
• make visual relevance and screenshot scenarios structural in managed plans and PR schema v3
• store exact PNG evidence, hashes, privacy receipts, and capability state outside Git history
• bind evidence freshness to commits, product diffs, scenarios, and PR preview fingerprints
• add browser-comment publication state, manual fallback, and fix-forward recovery
• cover managed and ad-hoc PR paths, configuration, documentation, troubleshooting, and release notes

Why

Frontend PR reviewers should be able to inspect a small set of exact screenshots without requiring projects to adopt a particular framework, committing large media files, or treating screenshots as mechanical proof of correctness.

Developer impact

The field remains backward compatible when omitted. Projects may opt into suggestions or make current evidence a publication requirement. Capture setup stays machine-local and must not edit application dependencies or configuration.

Validation

• go test ./...
• python3 -m unittest tests.test_product_loop (18 tests)
• git diff --check