js-on-sx: JSON.stringify + JSON.parse

Recursive-descent parser/serializer in SX.

stringify: type-of dispatch for primitives, lists, dicts. Strings
escape \\ \" \n \r \t.

parse: {:s src :i idx} state dict threaded through helpers.
Handles primitives, strings (with escapes), numbers, arrays,
objects.

Wired into js-global.

391/393 unit (+10), 148/148 slice unchanged.

plans/js-on-sx.md

@@ -187,6 +187,8 @@ Append-only record of completed iterations. Loop writes one line per iteration:
 
 - 2026-04-23 — **`String.fromCharCode`, `parseInt`, `parseFloat`.** `String` global with `fromCharCode` (variadic, loops through args and concatenates via `js-code-to-char`). `parseInt` truncates toward zero via `js-math-trunc`; `parseFloat` delegates to `js-to-number`. Wired into `js-global`. 5 new tests, **381/383** (376→+5). Conformance unchanged.
 
+- 2026-04-23 — **JSON.stringify + JSON.parse.** Shipped a recursive-descent parser and serializer in SX. `js-json-stringify` dispatches on `type-of` for primitives, lists, dicts. `js-json-parse` uses a state dict `{:s src :i idx}` mutated in-place by helpers (`js-json-skip-ws!`, `js-json-parse-value`, `-string`, `-number`, `-array`, `-object`). String parser handles `\n \t \r \" \\ \/` escapes. Number parser collects digits/signs/e+E/. then delegates to `js-to-number`. Array and object loops recursively call parse-value. JSON wired into `js-global`. 10 new tests (stringify primitives/arrays/objects, parse primitives/string/array/object), **391/393** (381→+10). Conformance unchanged.
+
 ## Phase 3-5 gotchas
 
 Worth remembering for later phases: