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/transpile.sx

@@ -254,15 +254,15 @@
   (fn
     (callee args)
     (cond
-      ((and (list? callee) (js-tag? callee "js-member"))
+      ((and (js-tag? callee "js-member") (not (js-has-spread? args)))
         (let
           ((recv (js-transpile (nth callee 1))) (key (nth callee 2)))
           (list
             (js-sym "js-invoke-method")
             recv
             key
-            (cons (js-sym "list") (map js-transpile args)))))
-      ((and (list? callee) (js-tag? callee "js-index"))
+            (js-transpile-args args))))
+      ((and (js-tag? callee "js-index") (not (js-has-spread? args)))
         (let
           ((recv (js-transpile (nth callee 1)))
             (key (js-transpile (nth callee 2))))
@@ -270,12 +270,29 @@
             (js-sym "js-invoke-method-dyn")
             recv
             key
-            (cons (js-sym "list") (map js-transpile args)))))
+            (js-transpile-args args))))
+      ((js-tag? callee "js-member")
+        (let
+          ((recv (js-transpile (nth callee 1))) (key (nth callee 2)))
+          (list
+            (js-sym "js-invoke-method")
+            recv
+            key
+            (js-transpile-args args))))
+      ((js-tag? callee "js-index")
+        (let
+          ((recv (js-transpile (nth callee 1)))
+            (key (js-transpile (nth callee 2))))
+          (list
+            (js-sym "js-invoke-method-dyn")
+            recv
+            key
+            (js-transpile-args args))))
       (else
         (list
           (js-sym "js-call-plain")
           (js-transpile callee)
-          (cons (js-sym "list") (map js-transpile args)))))))
+          (js-transpile-args args))))))
 
 ;; ── Array literal ─────────────────────────────────────────────────
 
@@ -295,10 +312,58 @@
 ;; order and allows computed values.
 (define
   js-transpile-array
-  (fn (elts) (cons (js-sym "list") (map js-transpile elts))))
+  (fn
+    (elts)
+    (if
+      (js-has-spread? elts)
+      (cons
+        (js-sym "js-array-spread-build")
+        (map
+          (fn
+            (e)
+            (if
+              (js-tag? e "js-spread")
+              (list (js-sym "list") "js-spread" (js-transpile (nth e 1)))
+              (list (js-sym "list") "js-value" (js-transpile e))))
+          elts))
+      (cons (js-sym "list") (map js-transpile elts)))))
 
 ;; ── Conditional ───────────────────────────────────────────────────
 
+(define
+  js-has-spread?
+  (fn
+    (lst)
+    (cond
+      ((empty? lst) false)
+      ((js-tag? (first lst) "js-spread") true)
+      (else (js-has-spread? (rest lst))))))
+
+;; ── Arrow function ────────────────────────────────────────────────
+
+(define
+  js-transpile-args
+  (fn
+    (args)
+    (if
+      (js-has-spread? args)
+      (cons
+        (js-sym "js-array-spread-build")
+        (map
+          (fn
+            (e)
+            (if
+              (js-tag? e "js-spread")
+              (list (js-sym "list") "js-spread" (js-transpile (nth e 1)))
+              (list (js-sym "list") "js-value" (js-transpile e))))
+          args))
+      (cons (js-sym "list") (map js-transpile args)))))
+
+;; ── Assignment ────────────────────────────────────────────────────
+
+;; `a = b` on an ident → (set! a b).
+;; `a += b` on an ident → (set! a (js-add a b)).
+;; `obj.k = v` / `obj[k] = v` → (js-set-prop obj "k" v).
 (define
   js-transpile-object
   (fn
@@ -320,8 +385,6 @@
             entries)
           (list (js-sym "_obj")))))))
 
-;; ── Arrow function ────────────────────────────────────────────────
-
 (define
   js-transpile-cond
   (fn
@@ -332,11 +395,6 @@
       (js-transpile t)
       (js-transpile f))))
 
-;; ── Assignment ────────────────────────────────────────────────────
-
-;; `a = b` on an ident → (set! a b).
-;; `a += b` on an ident → (set! a (js-add a b)).
-;; `obj.k = v` / `obj[k] = v` → (js-set-prop obj "k" v).
 (define
   js-transpile-arrow
   (fn
@@ -367,6 +425,9 @@
                 (append inits (list (js-transpile body))))))))
       (list (js-sym "fn") param-syms body-tr))))
 
+;; ── End-to-end entry points ───────────────────────────────────────
+
+;; Transpile + eval a single JS expression string.
 (define
   js-transpile-tpl
   (fn
@@ -378,6 +439,8 @@
       (else
         (cons (js-sym "js-template-concat") (js-transpile-tpl-parts parts))))))
 
+;; Transpile a JS expression string to SX source text (for inspection
+;; in tests). Useful for asserting the exact emitted tree.
 (define
   js-transpile-tpl-parts
   (fn
@@ -389,9 +452,6 @@
         (js-transpile (first parts))
         (js-transpile-tpl-parts (rest parts))))))
 
-;; ── End-to-end entry points ───────────────────────────────────────
-
-;; Transpile + eval a single JS expression string.
 (define
   js-transpile-assign
   (fn
@@ -429,8 +489,6 @@
             (js-transpile rhs))))
       (else (error "js-transpile-assign: unsupported target")))))
 
-;; Transpile a JS expression string to SX source text (for inspection
-;; in tests). Useful for asserting the exact emitted tree.
 (define
   js-compound-update
   (fn