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.

lib/js/test.sh

@@ -993,6 +993,28 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 1904)
 (eval "(js-eval \"parseFloat('3.14')\")")
 
+;; ── Phase 11.json: JSON.stringify / JSON.parse ────────────────
+(epoch 2000)
+(eval "(js-eval \"JSON.stringify(42)\")")
+(epoch 2001)
+(eval "(js-eval \"JSON.stringify('hi')\")")
+(epoch 2002)
+(eval "(js-eval \"JSON.stringify([1,2,3])\")")
+(epoch 2003)
+(eval "(js-eval \"JSON.stringify({a:1})\")")
+(epoch 2004)
+(eval "(js-eval \"JSON.stringify(true)\")")
+(epoch 2005)
+(eval "(js-eval \"JSON.parse('42')\")")
+(epoch 2006)
+(eval "(js-eval \"JSON.parse('true')\")")
+(epoch 2007)
+(eval "(js-eval \"JSON.parse('\\\"hello\\\"')\")")
+(epoch 2008)
+(eval "(js-eval \"JSON.parse('[1,2,3]').length\")")
+(epoch 2009)
+(eval "(js-eval \"JSON.parse('{\\\"a\\\":1}').a\")")
+
 EPOCHS
 
 
@@ -1525,6 +1547,18 @@ check 1902 "parseInt('42')"                 '42'
 check 1903 "parseInt(3.7)"                  '3'
 check 1904 "parseFloat('3.14')"             '3.14'
 
+# ── Phase 11.json ──────────────────────────────────────────────
+check 2000 "stringify(42)"                  '"42"'
+check 2001 "stringify('hi')"                '"\"hi\""'
+check 2002 "stringify array"                '"[1,2,3]"'
+check 2003 "stringify object"               '"{\"a\":1}"'
+check 2004 "stringify true"                 '"true"'
+check 2005 "parse 42"                       '42'
+check 2006 "parse true"                     'true'
+check 2007 "parse string"                   '"hello"'
+check 2008 "parse array length"             '3'
+check 2009 "parse object.a"                 '1'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "✓ $PASS/$TOTAL JS-on-SX tests passed"