feat: add the declarative .oven system and React oven kit

[apresmoi]

Summary

• Componentize all five ovens into a shared React component library (dashboard/src/oven/) at 100% visual and behavioral parity with the previous UI.
• Add the declarative .oven system: an XML authoring format compiled to a frozen IR and rendered by a shared reducer-backed OvenRuntime. All five ovens (Checklist, Streaming Diff, Differential Testing, Visual Parity, Performance Tracing) render live from a .oven at byte-identical DOM parity.
• Minimize the oven footprint: .ir.json is generated at build time (no longer committed), detail.json is removed via a schema-v5 oven migration (legacy runs still read), and each oven is now <id>.oven + instructions.md + a thin engine data layer.
• Support consumer-authored custom ovens end to end: burnlist oven create/fork plus a generic dashboard render path that serves and renders a bound custom oven's data.
• Add onboarding and authoring docs to the skill and website: a start-here mental-model guide, a .oven DSL reference, and a "designing ovens: measure what you can't fake" guide. The generated skill.md leads with concepts, setup, and honest-measurement.

Validation

• npm run build:dashboard, npm run verify, and npm run verify:package pass on Node 18.20.8, 20.19.6, and 22.
• DOM goldens are byte-identical and timezone-independent; the five ovens' rendered DOM is unchanged through the migration.
• cd website && npm run build passes.
• Live-integration and edge cases (compact bundles, 304 Not Modified polling, oven fork ids, switch default cases, malformed input) have dedicated behavioral tests alongside the DOM goldens.

Structure

• Roughly 16.7k of the changed lines are DOM goldens (byte-frozen render snapshots under **/goldens/ and *.golden.html).
• Read order: dashboard/src/oven/ (the React oven kit) → src/ovens/dsl/ (the .oven XML → IR compiler) → dashboard/src/oven/runtime/ (OvenRuntime, reducer, collection pipeline, adapters, poll transport) → ovens/<id>/ (the five .oven files + thin engine data layer) → src/server/ + src/cli/ (custom-oven discovery, create/fork, generic render + data serving) → skills/burnlist/ + website/ (skill, docs, onboarding).
• The DT vanilla renderer (dashboard/src/oven/differential-testing-render/*.js), the DT engine, and the dashboard server are pre-existing files relocated during the migration; they back the frozen golden oracle (AGENTS.md names them as decomposition targets). Newly-authored logic files are ≤400 lines.

Notes

• Root dependencies stays empty (zero runtime dependencies); the engine layer stays .mjs.