Phase 4 kernel-shrink seam. jolt-core/clojure/core.clj holds the Clojure portion
of clojure.core; api/init loads it into the clojure.core namespace after
init-core! interns the Janet primitives, routed through eval-toplevel so it
compiles via the self-hosted pipeline (or interprets when :compile? is off).
First three fns moved off the Janet side: ffirst, fnext, nnext (leaf, no internal
callers). Same results compiled or interpreted; full suite green; battery holds
3913.
