C4 models as code

"Diagrams as code", as featured on the ThoughtWorks Tech Radar, is becoming a popular way to create software architecture diagrams for inclusion in long-lived documentation. The benefits of creating diagrams from text are well understood - text is version controllable, easy to diff, easy to integrate into build pipelines, and automatic layout facilities allow authors to focus on content. The majority of these tools (e.g. PlantUML and Mermaid) are focussed on diagramming though, and require you to create one text file per diagram, with only a limited ability to reuse diagram elements across multiple diagrams. At scale, this causes maintenance problems, and leads to inconsistencies between diagrams.