docs: add enterprise readiness assessment and v4.0 roadmap

- Create ENTERPRISE_ASSESSMENT.md with comprehensive gap analysis
  benchmarking against Salesforce, ServiceNow, SAP, Dynamics 365
- Identify 23 critical gaps and 31 enhancement opportunities
- Define 6 implementation phases (12-17) for v4.0 enterprise readiness
- Update ROADMAP.md with v4.0 phases, timeline, and success criteria

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot