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.