Add bounded recipe step continuation

[chubes4]

Summary

• add finite, declarative continuation for JSON-producing recipe steps
• evaluate RFC 6901 predicates and map opaque result fields into subsequent invocations
• retain bounded digest/preview evidence on completion, failure, exhaustion, and timeout
• reject continuation for host-executed recipe helpers whose dispatch semantics differ

Closes #2218.

Verification

• npm run build
• npm run test:recipe-step-continuation
• npx tsx tests/recipe-step-metadata.test.ts
• npx tsx tests/recipe-validation-descriptors.test.ts
• exercised against a real resumable WordPress ability workload; the runner performed bounded repeated invocations and failed closed at the configured limit

AI Assistance

OpenAI gpt-5.6-sol via OpenCode was used to inspect the repository, implement the continuation contract, add tests, and run verification. Chris Huber reviewed and remains responsible for the change.