Add optional boolean security_boundary property to indicate whether a
node represents a security boundary in threat model diagrams.

- Add security_boundary to Cell.data.properties in OpenAPI spec
- Add security_boundary as required top-level property in MinimalNode
- Update transformNodeToMinimal() with shape-based enforcement rules:
  - security-boundary shape: always true
  - text-box shape: always false
  - other shapes: use data value or default false
- Include security_boundary in GraphML export

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>