Document schema and versioning

interface DesignDocumentV6 {
  schema: string;
  version: 6;
  name: string;
  units: { length: "mm"; angle: "rad"; mass?: "kg" };
  parameters: Record<string, ParameterIR>;
  materials?: Record<string, MaterialDefinitionIR>;
  configurations?: Record<string, DesignConfigurationIR>;
  topologyReferences?: Record<string, TopologyReferenceEntryIR>;
  nodes: Record<string, NodeIR>;
  outputs: Record<string, RefIR>;
  metadata?: Record<string, JsonValue>;
}