js-on-sx: spread ... in array literals and call args

Parser: jp-array-loop and jp-call-args-loop detect punct "..."
and emit (js-spread inner).

Transpile: when any element is spread, build array/args via
js-array-spread-build with (list "js-value" v) and (list
"js-spread" xs) tags.

Runtime: js-array-spread-build walks items, appending values or
splicing spreads via js-iterable-to-list (handles list/string/dict).

Works in arrays, call args, variadic fns (Math.max(...arr)),
and string spread ([...'abc']).

414/416 unit (+5), 148/148 slice unchanged.

lib/js/parser.sx

@@ -486,11 +486,18 @@
     (cond
       ((jp-at? st "punct" "]") nil)
       (else
-        (do
-          (append! elems (jp-parse-assignment st))
+        (begin
+          (cond
+            ((jp-at? st "punct" "...")
+              (begin
+                (jp-advance! st)
+                (append!
+                  elems
+                  (list (quote js-spread) (jp-parse-assignment st)))))
+            (else (append! elems (jp-parse-assignment st))))
           (cond
             ((jp-at? st "punct" ",")
-              (do (jp-advance! st) (jp-array-loop st elems)))
+              (begin (jp-advance! st) (jp-array-loop st elems)))
             (else nil)))))))
 
 ;; ── Entry point ─────────────────────────────────────────────────
@@ -600,11 +607,18 @@
     (cond
       ((jp-at? st "punct" ")") nil)
       (else
-        (do
-          (append! args (jp-parse-assignment st))
+        (begin
+          (cond
+            ((jp-at? st "punct" "...")
+              (begin
+                (jp-advance! st)
+                (append!
+                  args
+                  (list (quote js-spread) (jp-parse-assignment st)))))
+            (else (append! args (jp-parse-assignment st))))
           (cond
             ((jp-at? st "punct" ",")
-              (do (jp-advance! st) (jp-call-args-loop st args)))
+              (begin (jp-advance! st) (jp-call-args-loop st args)))
             (else nil)))))))
 
 (define