Continue the kernel shrink. last/butlast become canonical Clojure defs
(pure first/next/loop/recur) in the core.clj overlay, dropping two
realize-for-iteration uses from the Janet kernel.
second was the obvious next candidate but can't move: the self-hosted
compiler front-end (analyzer.clj) calls it, and the compiler has to
compile the overlay, so anything it uses must already exist as a Janet
primitive. Documented that as a third clause of the safe-to-move rule.
Suite green, clojure-test-suite battery holds 3913 pass / 58 clean,
binary builds and runs the embedded overlay.
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.
