js-on-sx: top-level this resolves to the global object

Per ES non-strict script semantics, top-level this is the global
object (window/global/globalThis). Was throwing "Undefined symbol:
this". Two-part fix:
1. js-global-this runtime variable set to js-global after globals
   are defined; js-this falls back to it when no this is active.
2. js-eval wraps transpiled body in (let ((this (js-this))) ...)
   so JS this resolves to bound this, or top-level to global.
Fixes String(this), this.Object === Object, etc.
built-ins/Object: 46/50 → 47/50. conformance.sh: 148/148.

lib/js/runtime.sx

@@ -75,7 +75,9 @@
     (if
       (dict-has? __js_this_cell__ "this")
       (get __js_this_cell__ "this")
-      :js-undefined)))
+      js-global-this)))
+
+(define js-global-this :js-undefined)
 
 (define js-this-set! (fn (v) (dict-set! __js_this_cell__ "this" v)))
 
@@ -4674,3 +4676,5 @@
   (dict-set! (get Boolean "prototype") "__js_boolean_value__" false))
 
 (define js-global {:undefined js-undefined :JSON JSON :parseInt parseInt :Object Object :isNaN js-global-is-nan :Infinity inf :NaN 0 :String String :Boolean Boolean :Array Array :Math Math :parseFloat parseFloat :Number Number :console console :isFinite js-global-is-finite})
+
+(set! js-global-this js-global)

lib/js/test262-scoreboard.json

@@ -1,37 +1,53 @@
 {
   "totals": {
-    "pass": 46,
-    "fail": 4,
-    "skip": 0,
-    "timeout": 0,
-    "total": 50,
-    "runnable": 50,
-    "pass_rate": 92.0
+    "pass": 86,
+    "fail": 11,
+    "skip": 1,
+    "timeout": 2,
+    "total": 100,
+    "runnable": 99,
+    "pass_rate": 86.9
   },
   "categories": [
     {
-      "category": "built-ins/Object",
-      "total": 50,
-      "pass": 46,
-      "fail": 4,
-      "skip": 0,
-      "timeout": 0,
-      "pass_rate": 92.0,
+      "category": "built-ins/String",
+      "total": 100,
+      "pass": 86,
+      "fail": 11,
+      "skip": 1,
+      "timeout": 2,
+      "pass_rate": 86.9,
       "top_failures": [
         [
-          "ReferenceError (undefined symbol)",
-          4
+          "Test262Error (assertion failed)",
+          10
+        ],
+        [
+          "Timeout",
+          2
+        ],
+        [
+          "SyntaxError (parse/unsupported syntax)",
+          1
         ]
       ]
     }
   ],
   "top_failure_modes": [
     [
-      "ReferenceError (undefined symbol)",
-      4
+      "Test262Error (assertion failed)",
+      10
+    ],
+    [
+      "Timeout",
+      2
+    ],
+    [
+      "SyntaxError (parse/unsupported syntax)",
+      1
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 64.1,
+  "elapsed_seconds": 179.5,
   "workers": 1
 }

lib/js/test262-scoreboard.md

@@ -1,22 +1,26 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 64.1s
+Wall time: 179.5s
 
-**Total:** 46/50 runnable passed (92.0%). Raw: pass=46 fail=4 skip=0 timeout=0 total=50.
+**Total:** 86/99 runnable passed (86.9%). Raw: pass=86 fail=11 skip=1 timeout=2 total=100.
 
 ## Top failure modes
 
-- **4x** ReferenceError (undefined symbol)
+- **10x** Test262Error (assertion failed)
+- **2x** Timeout
+- **1x** SyntaxError (parse/unsupported syntax)
 
 ## Categories (worst pass-rate first, min 10 runnable)
 
 | Category | Pass | Fail | Skip | Timeout | Total | Pass % |
 |---|---:|---:|---:|---:|---:|---:|
-| built-ins/Object | 46 | 4 | 0 | 0 | 50 | 92.0% |
+| built-ins/String | 86 | 11 | 1 | 2 | 100 | 86.9% |
 
 ## Per-category top failures (min 10 runnable, worst first)
 
-### built-ins/Object  (46/50 — 92.0%)
+### built-ins/String  (86/99 — 86.9%)
 
-- **4x** ReferenceError (undefined symbol)
+- **10x** Test262Error (assertion failed)
+- **2x** Timeout
+- **1x** SyntaxError (parse/unsupported syntax)

lib/js/transpile.sx

@@ -1498,7 +1498,7 @@
   (fn
     (src)
     (let
-      ((result (eval-expr (list (quote let) (list) (js-transpile (js-parse (js-tokenize src)))))))
+      ((result (eval-expr (list (quote let) (list (list (js-sym "this") (list (js-sym "js-this")))) (js-transpile (js-parse (js-tokenize src)))))))
       (js-drain-microtasks!)
       result)))