js-on-sx: Object.assign uses js-set-prop so keys appear in __js_order__

lib/js/runtime.sx

@@ -4518,7 +4518,7 @@
                         (for-each
                           (fn
                             (k)
-                            (if (js-key-internal? k) nil (dict-set! target k (get src k))))
+                            (if (js-key-internal? k) nil (js-set-prop target k (get src k))))
                           (js-object-keys src)))
                       ((= (type-of src) "string")
                         (let
@@ -4535,7 +4535,7 @@
       ((>= i n) nil)
       (else
         (begin
-          (dict-set! target (js-to-string i) (char-at s i))
+          (js-set-prop target (js-to-string i) (char-at s i))
           (js-object-assign-string-loop target s (+ i 1) n))))))
 
 (define js-object-freeze (fn (o) o))

plans/js-on-sx.md

@@ -158,6 +158,8 @@ Each item: implement → tests → update progress. Mark `[x]` when tests green.
 
 Append-only record of completed iterations. Loop writes one line per iteration: date, what was done, test count delta.
 
+- 2026-05-10 — **`Object.assign` keys now visible to `Object.keys` / `JSON.stringify`.** `Object.assign({}, {a:1})` was mutating the target via `dict-set!` which bypasses our `__js_order__` insertion-order side table; `Object.keys(t)` (which iterates `__js_order__` when present) returned `[]`, and `JSON.stringify` saw nothing. Switched `js-object-assign` to use `js-set-prop` (which calls `js-obj-order-add!` on new keys) for both dict and string sources. Result: built-ins/Object/assign 13/25 → 14/25. conformance.sh: 148/148.
+
 - 2026-05-10 — **User functions' `prototype` chain through Object.prototype + auto-set `constructor`.** Per ES spec, every function's `prototype` slot defaults to `{ constructor: F, __proto__: Object.prototype }`. Our `js-get-ctor-proto` lazily created a fresh empty `(dict)` for user functions on first access — so `(new F) instanceof Object` was `false`, `F.prototype.constructor` was undefined, and `x.constructor === F` failed. Now the lazy-init seeds the proto with `__proto__ → Object.prototype` and `constructor → F` before caching in `__js_proto_table__`. Result: language/expressions/instanceof 25/30 → 26/30. conformance.sh: 148/148.
 
 - 2026-05-10 — **Postfix `++`/`--` reject a preceding LineTerminator (ASI).** Per ES spec, `x\n++;` is a syntax error: no LineTerminator allowed between LHS and postfix `++`/`--`. Our `jp-parse-postfix` was matching `++`/`--` regardless of whether the preceding token had `:nl true`. Added `(not (jp-token-nl? st))` guard so newline-before-`++` makes the postfix arm fall through, the `++` then becomes a prefix-expr starting a new statement, which fails to parse and the runner classifies as SyntaxError. Result: language/expressions/postfix-increment 16/30 → 18/30 (+2). postfix-decrement 16/30 → 18/30 (+2). conformance.sh: 148/148.