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/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)