feat: add generic Oven progress events

[alowpoly]

Summary

• add the generic burnlist-oven-event@1 contract and append-only monotonic per-Oven event store
• expose bounded JSON and SSE replay with vector cursors, stream caps, backpressure, and per-Oven corruption fail-close (a corrupt record fail-closes that Oven's stream tail, isolated to the one Oven; other Oven streams are unaffected)
• publish Checklist and Differential Testing observations only after canonical state is durable
• export the event API and CLI while keeping coordination guidance in the single shipped Burnlist skill

Validation

• npm run verify
• npm run verify:package
• skill quick validation
• git diff --check

Notes

• follow-up to feat: add the declarative .oven system and React oven kit #13
• retained storage remains append-only; bounded reads preserve deterministic idempotency