> The easiest way to do that, in my experience, is to have a manifest in every repo that specifies dependencies as a flat list, which the CD system can parse to build a dependency graph.

Thats pretty much how we manage the contracts.

> To solve this problem, your CD system needs to automatically trigger rebuilds (and re-tests) of dependent (downstream) services when a dependency (upstream) changes

This is GOLD. I think you may have just solved the problem with something so obviously simple that it has made me feel like a fool... Many thanks!!!