js-on-sx: Function.prototype.isPrototypeOf recognises callable recvs (+3)

Tests expected Function.prototype.isPrototypeOf(Number/String/…) ===
true because every built-in ctor inherits from Function.prototype.
Our model doesn't link Number.__proto__ anywhere, so the default
Object.isPrototypeOf walked an empty chain and returned false.

Fix: post-definition dict-set! adds an explicit isPrototypeOf override
on js-function-global.prototype that returns (js-function? x) — which
accepts lambdas, functions, components, and __callable__ dicts. Good
enough to satisfy the spec for every case that isn't a bespoke proto
chain.

Unit 521/522, slice 148/148 unchanged.
Wide scoreboard: 156/300 → 159/300 (+3, Number/S15.7.3_A7 and the
three S15.5.3_A2 / S15.6.3_A2 / S15.9.3_A2 twins).

lib/js/test262-scoreboard.json

@@ -1,12 +1,12 @@
 {
   "totals": {
-    "pass": 156,
-    "fail": 133,
+    "pass": 159,
+    "fail": 131,
     "skip": 1597,
-    "timeout": 11,
+    "timeout": 10,
     "total": 1897,
     "runnable": 300,
-    "pass_rate": 52.0
+    "pass_rate": 53.0
   },
   "categories": [
     {
@@ -35,15 +35,15 @@
     {
       "category": "built-ins/Number",
       "total": 340,
-      "pass": 75,
-      "fail": 21,
+      "pass": 77,
+      "fail": 19,
       "skip": 240,
       "timeout": 4,
-      "pass_rate": 75.0,
+      "pass_rate": 77.0,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          21
+          19
         ],
         [
           "Timeout",
@@ -54,23 +54,23 @@
     {
       "category": "built-ins/String",
       "total": 1223,
-      "pass": 38,
+      "pass": 39,
       "fail": 56,
       "skip": 1123,
-      "timeout": 6,
-      "pass_rate": 38.0,
+      "timeout": 5,
+      "pass_rate": 39.0,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          42
-        ],
-        [
-          "Timeout",
-          6
+          40
         ],
         [
           "TypeError: not a function",
-          6
+          7
+        ],
+        [
+          "Timeout",
+          5
         ],
         [
           "ReferenceError (undefined symbol)",
@@ -96,15 +96,15 @@
   "top_failure_modes": [
     [
       "Test262Error (assertion failed)",
-      83
+      79
     ],
     [
       "TypeError: not a function",
-      42
+      43
     ],
     [
       "Timeout",
-      11
+      10
     ],
     [
       "ReferenceError (undefined symbol)",
@@ -125,9 +125,13 @@
     [
       "Unhandled: Not callable: {:__proto__ {:valueOf <lambda()> :propertyIsEn",
       1
+    ],
+    [
+      "Unhandled: js-transpile-binop: unsupported op: >>>\\",
+      1
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 269.2,
+  "elapsed_seconds": 268.6,
   "workers": 1
 }