- Upgrade sequence diagram to Option A: unified SQL+Mongo flow, numbered
  steps, shows audit on both happy + reject paths, statement_timeout
  visible, per-conn vs global QPM distinction
- Render to PNG via mermaid-cli with dark theme matching v2 images
  (#0f172a bg + purple #6e56cf accents)
- README shows PNG by default; Mermaid source kept in <details> block
  for contributors, plus regen command
- New files: docs/images/query-flow.{png,mmd}, mermaid-config.json
- Bump 0.4.2 -> 0.4.3