Number.prototype.constructor === Number etc. Four dict-set! lines add
the backlink after each constructor dict is defined.
new String().constructor === String now returns true. Array literals
don't yet link to Array.prototype so [].constructor === Array is still
false — that would need a boxing refactor.
Unit 521/522, slice 148/148 unchanged.
Number 76/100 → 77/100 (+1). String variance-heavy under CPU load.