js-on-sx: js-display filter to format JS values without leaking internals

The OCaml epoch-protocol printer serializes raw SX dicts. JS object literals
now carry __proto__ / __js_order__ bookkeeping that points into Object.prototype,
a complex dict containing lambdas that close over Object — the printer
recurses indefinitely and hangs.

js-display walks the value once, dropping any dict key that matches the
__name__ dunder convention. js-eval calls it on its return value so the
output is the user-facing shape only. Restores 587/593 passing (up from
191/593 post-merge and 492/585 pre-merge) — the surviving 6 failures are
legitimate pre-existing test mismatches (illegal return/break/continue,
parseFloat float vs rational, escaped backtick).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/js/transpile.sx

@@ -1593,6 +1593,36 @@
         param-syms
         (list (js-sym "js-async-wrap") (list (js-sym "fn") (list) body-tr))))))
 
+(define
+  js-internal-key?
+  (fn
+    (k)
+    (and
+      (string? k)
+      (>= (len k) 4)
+      (= (substring k 0 2) "__")
+      (= (substring k (- (len k) 2) (len k)) "__"))))
+
+(define
+  js-display
+  (fn
+    (val)
+    (cond
+      ((dict? val)
+        (let
+          ((cleaned (dict)))
+          (begin
+            (for-each
+              (fn
+                (k)
+                (when
+                  (not (js-internal-key? k))
+                  (dict-set! cleaned k (js-display (get val k)))))
+              (keys val))
+            cleaned)))
+      ((list? val) (map js-display val))
+      (else val))))
+
 (define
   js-eval
   (fn
@@ -1600,6 +1630,6 @@
     (let
       ((result (eval-expr (list (quote let) (list (list (js-sym "this") (list (js-sym "js-this")))) (js-transpile (js-parse (js-tokenize src)))))))
       (js-drain-microtasks!)
-      result)))
+      (js-display result))))
 
 (define js-compile-expr (fn (src) (js-transpile (js-parse-expr src))))