How do you measure architecture drift?
You need a machine-readable statement of the intended architecture and a map derived from the actual code, then you compare them. Useful measures are the count of dependency-rule violations, components in the code with no counterpart in the design, and components in the design with nothing implementing them.
From Architecture drift: what it is and how to catch it, published 9 August 2026. That article is where the reasoning behind this answer is set out, including what it does not cover.