Add Dynu Terraform brownfield DNS documentation layer

docs/source-of-truth.md

@@ -30,6 +30,10 @@ Use Terraform when documenting/reconciling existing:
 
 Do **not** treat Terraform as a full replacement for Compose operations in this repo.
 
+- Dynu public DNS records remain authoritative at Dynu.
+- Terraform Dynu configuration mirrors/reconciles Dynu DNS state for documentation and controlled drift management.
+- Imported Dynu Terraform state reflects actual provider-side DNS state at import time.
+
 
 ### Ansible bootstrap decisions