Add challenge withdrawal reimbursement guard

[AlonePenguin]

@algora-pbc /claim #18
/claim #18

Summary

• Adds challenge-withdrawal-reimbursement-guard/, a dependency-free closeout guard for sponsor withdrawals, cancellations, and material scope reductions after solvers have started work.
• Computes deterministic solver reimbursement recommendations from milestone progress, documented non-refundable costs, bounty caps, and sponsor-funded reserves.
• Flags cancellation authority gaps, unequal solver notices, missing appeal/cost-claim instructions, short dispute holds, missing evidence, reimbursement shortfalls, reserve shortfalls, and incomplete IP/data return attestations.
• Includes clean/risky synthetic fixtures, JSON and Markdown reports, an SVG dashboard, and a committed MP4 demo artifact.

Issue Fit

This is scoped to Scientific Bounty System issue #18 and is distinct from the existing intake, rubric readiness, scoring/arbitration, appeals, escrow, payout eligibility/routing, sponsor compliance, debrief feedback, onboarding clock parity, communication parity, accessibility/localization, deadline fairness, and amendment-consent slices. It focuses specifically on cancellation closeout fairness and solver-cost reimbursement after teams have already incurred work.

Validation

npm --prefix challenge-withdrawal-reimbursement-guard run check
npm --prefix challenge-withdrawal-reimbursement-guard run demo
npm --prefix challenge-withdrawal-reimbursement-guard run make-demo-video
npm --prefix challenge-withdrawal-reimbursement-guard run verify-video

Observed locally:

challenge withdrawal reimbursement guard tests passed
Wrote challenge withdrawal reimbursement guard reports:
- challenge-withdrawal-reimbursement-guard/reports/clean-withdrawal-report.json
- challenge-withdrawal-reimbursement-guard/reports/risky-withdrawal-report.json
- challenge-withdrawal-reimbursement-guard/reports/risky-withdrawal-handoff.md
- challenge-withdrawal-reimbursement-guard/reports/withdrawal-dashboard.svg
Wrote challenge-withdrawal-reimbursement-guard/reports/demo.mp4
demo.mp4 verified: h264, 960x540, 4.000s, 18/1