Add settlement confidence assistant

[Ani-107]

Summary

Adds a Settlement Confidence Assistant for Greenroom's settlement workflow. The assistant interprets messy deal notes, surfaces ambiguity/confidence gaps, and explains settlement line items with source-backed reasoning.

Why this slice

The core settlement problem is not just calculation. Mariana can calculate settlements in a spreadsheet; the harder problem is trust, interpretation, and dispute prevention when prose, structured fields, recoups, and sign-off status drift apart.

What changed

• Added a lightweight AI-style interpretation layer in lib/settlementConfidence.ts
• Added an inline confidence assistant panel to /shows/[id]/settle
• Flags conflicts such as status/sign-off mismatch, unresolved disputed recoups, structured field drift, and expense cap issues
• Explains key settlement lines with source text and confidence
• Added PRD memo and supporting case-study artifacts under docs/

Validation

• npm run build passes
• Demo path after running locally:
  http://localhost:3000/shows/show_coastal_spell_dispute/settle