Add concise architecture diagram

[lraigosov]

Summary

• add a concise Mermaid architecture diagram to the README
• keep the detailed architecture diagram as a second level of reference
• improve quick onboarding by showing the main layers of the lab in a simpler view

Validation

• reviewed the diff between origin/main and origin/dev
• confirmed the change only affects README.md
• validated Mermaid syntax successfully
• confirmed Mermaid preview renders correctly from README

Impact

• documentation-only change
• no Docker Compose, runtime, or startup policy changes