Add challenge solver withdrawal continuity guard

[haki203]

Summary

• Adds a distinct challenge-solver-withdrawal-continuity-guard/ slice for Scientific Bounty System issue Scientific Bounty System #18.
• Evaluates active solver/team withdrawals or replacements after private challenge workspace access has started.
• Holds unsafe continuity when withdrawal notice, workspace access revocation, team quorum, IP return, artifact custody, milestone continuity, or sponsor/reviewer notifications are incomplete.
• Emits deterministic JSON, Markdown, SVG, and MP4 reviewer artifacts from synthetic data only.

Non-overlap

This is not another intake, rubric readiness, submission privacy, solver prequalification, data-room access, scoring/arbitration, appeals, anti-collusion, escrow settlement, payout eligibility, sponsor reliability, amendment consent, reviewer consensus, IP redaction, milestone progress, evidence freeze, clarification freeze, award transparency, benchmark leakage, deliverable acceptance, reproducibility environment, license/dependency, human-subjects, or cancellation/no-award slice. It focuses specifically on solver withdrawal and replacement continuity after work has started.

Validation

• node --test challenge-solver-withdrawal-continuity-guard/test.js -> 3 tests passed
• FFMPEG_PATH=<local ffmpeg-static binary> node challenge-solver-withdrawal-continuity-guard/demo.js -> generated JSON/Markdown/SVG/MP4 reviewer artifacts
• node --check challenge-solver-withdrawal-continuity-guard/index.js
• node --check challenge-solver-withdrawal-continuity-guard/sample-data.js
• node --check challenge-solver-withdrawal-continuity-guard/demo.js
• node --check challenge-solver-withdrawal-continuity-guard/test.js
• ffmpeg -v error -i challenge-solver-withdrawal-continuity-guard/reports/demo.mp4 -f null - -> decoded without errors
• git diff --cached --check

Synthetic data only. No credentials, private challenge files, payment processor calls, bank/ACH/wallet actions, KYC, payout systems, sponsor data rooms, live workspaces, or external APIs are used.

/claim #18