feat(feed): establish Codex rendering foundation

[Juliusolsson05]

Summary

• add a first-principles feed render model for committed transcript + live semantic ownership
• fix Codex semantic replacement when completed proxy turns remain unsealed
• suppress committed assistant duplicates even when rollout rows lack message ids
• add render-diagnostics.json to debug bundles and capture rendering research docs

Verification

• npm run test:feed-render-model
• npm run test:semantic-committed-text
• npm run test:semantic-fold-codex-replace
• npm run build