Fourth pure-fn batch (jolt-1j0 phase 2), 6 leaf fns. distinct? now uses
value-semantics (the prior Janet impl keyed a table by identity, so equal
collections compared distinct); nthnext uses the canonical loop form, which
fixes (nthnext nil nil) => nil and the nil-count cases the prior pre-check threw
on. replace preserves nil values via get-with-default.
conformance 218/218 x3, clojure-test-suite 3927 -> 3928 (canonical nthnext),
core-bench A/B flat. Per-batch gate caught a transient -1 from the first
nthnext rewrite; fixed before commit.
4aec21e603