[C5] Validator: Graph Checks

[kumaakh]

Sprint 3 | P3 Low | 1 day

Add graph structure validation: DAG check, orphan detection.

📄 Full Specification

docs/declarative-pipeline/tasks/C2-C5-validator-enhancements.md (Section C5)

Acceptance Criteria

• No source → Error
• Cycle detected → Error
• Orphan module → Warning