feat: add product continuity contracts

[davidahmann]

Summary

• add source-backed product, specification, impact, and scenario contracts with generated schemas
• require item-level continuity evidence and bind worker execution to immutable admitted context
• make worker settlement, recovery, review, and remote readback fail closed without inventing success
• freeze the first Node 24, Next.js 16, React 19.2 web recipe decision while keeping recipe application out of this wave

Validation

• npm run check under repository-pinned Node 24.20.0
• 21 test files and 147 tests passed
• workflow, generated-schema, coverage, and packaged draft-PR lifecycle canaries passed
• exact-head local Codex review repairs completed; final redundant review process stalled only after independently rerunning the full green gate