feat(errors): standardize contract error taxonomy and client decoding strategy

[observerr411]

• Define top-level error categories: AUTH, VALIDATION, STATE, LIMIT, SYSTEM, INTEGRATION
• Establish numeric ranges / mapping strategy for consistent classification
• Map all existing contract errors and panics into the new taxonomy
• Document category definitions and intended usage
• Add client-side error decoding strategy to docs
• Include example client error-handling snippet

This closes #225