The ln -s … step allows me to run in development mode without problems. So I will leave it marked as the solution.
However, the existence of the /tmp/Rack2/libRack.dylib link seems to have broken my standard Rack installation (as opposed to my separate development mode installation). When that file is present, and I launch Rack the standard way, I don’t see any modules in my library (despite them being present in my user folder) and Rack offers to download them. I download them, restart Rack, and the same thing happens again: no modules in the browser, Rack offers to download them again. When I delete /tmp/Rack2/libRack.dylib the problem goes away.
So the steps I’ve taken to resolve my development mode problem have created new non-development mode problems, defeating the purpose of maintaining separate Rack installations for development and music making.
Maybe I’m outside the realm of what is supported by Rack. But I believe I’m using the --dev option as intended.