^one amendment after stu's suggestion: this could and probably should be using the LMDB store and the engine rust code to retrieve and materialize cache entries, instead of interacting with the v1 artifact cache. it's not 100% clear, but i was vaguely thinking an adaptor from v1 cache output to LMDB would be a necessary intermediate step, which should then make "porting to v2" a matter of removing all v1 tasks.