feat: add Visual Parity Oven and engine identity

[alowpoly]

Summary

• port Visual Parity onto the current built-in Oven registry and React dashboard
• validate isolated render domains, target-vs-context verdicts, calibrated tolerances, and retained screenshot triplets
• accept optional { id, runtimeRoot } engine identity in Differential Testing scenario catalogs and bundle transport
• keep existing Checklist, Performance Tracing, and Streaming Diff paths unchanged

Validation

• npm run build:dashboard
• npm run test:visual-parity
• npm run test:differential-testing
• npm run verify — 434 tests passed
• npm run verify:package

Context

PR #7 landed only on the retired stacked Streaming Diff branch. This ports the Oven directly onto current main without restoring the old render architecture or compatibility paths.