I've been thinking that the Scheme code (ac.scm) should be split in two parts: an "axiomatic" part that defines car, cdr, set, annotate, etc; and a "library" part. The "library" part would define OS stuff like current-gc-milliseconds, open-socket, probably atomic-invoke, etc. There would probably be a "math" library that defines exp, sqrt, and all the missing math functions.
This structure would make it clear what parts of the language are really axioms, and what parts are in Scheme for performance, convenience, or because they are OS things that require low-level hooks.
Here "scheme-fn" is a macro that returns an Arc function that calls the named Scheme function, but still expects to be passed Scheme values and returns a Scheme value. Then various functions such as "scheme-istrue" can be used to convert Arc values to Scheme values and back again.