Index unsafety w.r.t. bounds and `undefined` is something TS would like to fix; but it's not ergonomic to fix until there are, something like automatic refineme

Index unsafety w.r.t. bounds and `undefined` is something TS would like to fix; but it's not ergonomic to fix until there are, something like automatic refinement range types applied to array length fields. Without it, TS'd get in the way, demanding `undefined` checks it almost definitely didn't need. Those range types and their complexity are, ultimately, what currently block TS from making these indexed accesses safer.