fix(feed): suppress committed semantic assistant duplicates

[Juliusolsson05]

Summary

• suppress finalized semantic assistant text when the same assistant text is already committed to the transcript
• keep the existing turn-scoped suppression path, but add exact committed-text suppression for Codex resp_* vs rollout-id splits
• add a regression script based on the 2026-05-16 debug bundle duplicate shape

Testing

• npm run test:semantic-committed-text
• npm run test:ghost-fallback
• npm run build