Add busboy library security review and safe wrapper (closes #6)

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What

Security review of the busboy multipart parser library used by restfuncs, per issue #6.

Findings

• Busboy has zero side effects — no filesystem access, no network calls, no eval, no child_process
• Dependency tree is minimal: only streamsearch (pure Boyer-Moore-Horspool algorithm)
• Existing protections: header pair/header size limits (matches Node.js defaults), strict mode, input validation
• The parsing code is correct but uses low-level character arithmetic making it hard to audit visually

Deliverables

1. SECURITY_REVIEW.md — Full analysis of busboy architecture, dependencies, security hardening, and recommendations
2. safe-busboy.js — Drop-in wrapper that adds:
   • File size/type/count limits
   • Field size/count limits
   • Request timeout (30s default)
   • MIME type whitelist
   • All limits configurable

Verdict

Busboy is safe to use as-is for restfuncs. The wrapper provides defense-in-depth for production deployments.

Closes #6