js-on-sx: constructors carry __proto__ = Function.prototype

Object/Array/Number/String/Boolean had no __proto__, so
Function.prototype mutations were invisible to them. Added a
post-init (begin (dict-set! ...)) at the end of runtime.sx
that wires each constructor to js-function-global.prototype.
Combined with the recent Object.prototype fallback, the chain
now terminates correctly: ctor → Function.prototype → Object.prototype.
built-ins/Number: 41/50 → 42/50, built-ins/String: 75/99 → 78/99,
built-ins/Array: 12/45 → 13/45. conformance.sh: 148/148.

lib/js/test262-scoreboard.md

@@ -1,13 +1,13 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 250.0s
+Wall time: 273.6s
 
-**Total:** 77/99 runnable passed (77.8%). Raw: pass=77 fail=16 skip=1 timeout=6 total=100.
+**Total:** 78/99 runnable passed (78.8%). Raw: pass=78 fail=15 skip=1 timeout=6 total=100.
 
 ## Top failure modes
 
-- **14x** Test262Error (assertion failed)
+- **13x** Test262Error (assertion failed)
 - **6x** Timeout
 - **1x** ReferenceError (undefined symbol)
 - **1x** SyntaxError (parse/unsupported syntax)
@@ -16,13 +16,13 @@ Wall time: 250.0s
 
 | Category | Pass | Fail | Skip | Timeout | Total | Pass % |
 |---|---:|---:|---:|---:|---:|---:|
-| built-ins/String | 77 | 16 | 1 | 6 | 100 | 77.8% |
+| built-ins/String | 78 | 15 | 1 | 6 | 100 | 78.8% |
 
 ## Per-category top failures (min 10 runnable, worst first)
 
-### built-ins/String  (77/99 — 77.8%)
+### built-ins/String  (78/99 — 78.8%)
 
-- **14x** Test262Error (assertion failed)
+- **13x** Test262Error (assertion failed)
 - **6x** Timeout
 - **1x** ReferenceError (undefined symbol)
 - **1x** SyntaxError (parse/unsupported syntax)