Chez Phase 1 (increment 3n): seq-native shims + reduced

The dominant prelude-parity crash bucket was 'apply non-procedure jolt-nil':
core fns calling seed-native seq fns (core_coll.janet) that have no Chez RT
shim, so var-deref returns jolt-nil. A static scan of the assembled prelude
turned up 52 referenced-but-undefined clojure.core names.

host/chez/natives-seq.ss shims the safe seq fns over the existing seq layer:
mapcat, take-while, drop-while, partition (collection arities only — the 1-arg
transducer forms are jolt-kxsr), and sort (compare default; a comparator may
return a 3-way number or a boolean less-than). reduced/reduced? is a jolt-reduced
record in seq.ss that reduce short-circuits on and deref unwraps, so unreduced
works. identical? = jolt= (the seed's definition).

Deferred list?: a Chez lazy seq and a list are both cseq, so it can't be told
apart without a distinct list type — a real divergence risk.

Parity 1407 -> 1467/2497, 0 new divergences. emit-test 263/263.

host/chez/atoms.ss

@@ -17,10 +17,13 @@
 ;; (watches/validators are overlay features layered via jolt.host/ref-put!).
 (define (jolt-atom-new v . _opts) (make-jolt-atom v))
 
+;; deref reads an atom; it also unwraps a `reduced` (Clojure @(reduced x) => x,
+;; which the overlay's `unreduced` relies on). The reduced record is in seq.ss.
 (define (jolt-deref x)
-  (if (jolt-atom? x)
-      (jolt-atom-val x)
-      (error #f "deref: unsupported reference type" x)))
+  (cond
+    ((jolt-atom? x) (jolt-atom-val x))
+    ((jolt-reduced? x) (jolt-reduced-val x))
+    (else (error #f "deref: unsupported reference type" x))))
 
 ;; (swap! a f arg*) -> (reset! a (f @a arg*)); f is invoked through jolt-invoke
 ;; (a jolt fn value, keyword, or invokable collection).