js-on-sx: numeric keys in object literals stringify on parse

{0: 41, 1: 42} was raising 'dict-set!: dict key val' because the parser
kept numeric keys as numbers in the entry dict, but SX dicts require string
keys. Now we str-coerce number-type tokens during jp-parse-object-entry.
Unblocks a huge chunk of test262 array-like-receiver tests that build
{length: N, 0: v, 1: v, ...} literals.

3 new tests, 453/455 total.

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

lib/js/parser.sx

@@ -552,7 +552,7 @@
           (do
             (jp-advance! st)
             (jp-expect! st "punct" ":")
-            (append! kvs {:value (jp-parse-assignment st) :key (get t :value)})))
+            (append! kvs {:value (jp-parse-assignment st) :key (str (get t :value))})))
         ((= (get t :type) "keyword")
           (do
             (jp-advance! st)

lib/js/test.sh

@@ -1147,6 +1147,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 3301)
 (eval "(js-eval \"var o = {a:1, b:2}; delete o.a; o.b\")")
 
+;; ── Phase 11.numkey: numeric object-literal keys ─────────────
+(epoch 3350)
+(eval "(js-eval \"var a = {0: 41, 1: 42, 2: 43}; a[0]\")")
+(epoch 3351)
+(eval "(js-eval \"var a = {length: 3, 0: 10, 1: 20, 2: 30}; a.length\")")
+(epoch 3352)
+(eval "(js-eval \"var a = {0: 'x'}; a['0']\")")
+
 ;; ── Phase 11.fnmethod: Function.prototype.call/apply/bind ─────
 (epoch 3400)
 (eval "(js-eval \"function greet3(n) { return 'hi ' + n; } greet3.call(null, 'ada')\")")
@@ -1784,6 +1792,11 @@ check 3201 "obj multi rename"              '3'
 check 3300 "delete obj.x"                  'true'
 check 3301 "delete obj.a keeps b"         '2'
 
+# ── Phase 11.numkey: numeric object-literal keys ─────────────
+check 3350 "numeric key [0]"               '41'
+check 3351 "numkey with length"           '3'
+check 3352 "numeric key ['0']"             '"x"'
+
 # ── Phase 11.fnmethod: call/apply/bind ────────────────────────
 check 3400 "fn.call basic"                 '"hi ada"'
 check 3401 "fn.apply basic"                '"hi bob"'