Define WordPress state artifact contract

[chubes4]

Summary

• Documents the canonical wp-codebox/wordpress-state-export/v1 and wp-codebox/wordpress-state-section/v1 contracts for replayable generic WordPress state.
• Adds fixture smoke coverage for the export and section artifact shapes.
• Stabilizes section artifact versioning and media export fields so consumers can rely on portable state fields without per-consumer projection shims.

Closes #462.
Related context: #444.

Verification

• npm run wordpress-state-contract-smoke
• npm run build
• npm run runtime-episode-smoke passed once against WordPress Playground. A later rerun hit an environment port collision with an already-running Playground listener on 127.0.0.1:51723.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the contract documentation, fixture smoke coverage, and focused runtime artifact contract updates; Chris remains responsible for review and merge.