Bounds and errors

Reading xs[i].x with a bad i throws a TypeError in plain JavaScript (xs[i] is undefined). A typed array would quietly return undefined instead, so Viscosity keeps the original behaviour.

• Proven accesses (inside for (let i = 0; i < xs.length; i++), for (const p of xs), and similar loops, including outer-loop indexes and && conditions) compile to a plain xs.x[i].
• Other accesses compile to xs.x[__vsc_bc(xs, i, 'x', false)]. The helper returns i when it is in range and otherwise throws TypeError: Cannot read properties of undefined (reading 'x'), the same message V8 gives.

--bounds assume drops the checks. Use it only when you are sure every index is valid.