runtime: reading a field of an out-of-bounds array element returns undefined instead of throwing TypeError #7153

PR #7149 added test-files/test_gap_repsel_ptr_shape_elements.ts and had to
drop one case from it, because it is red on main:

class Row { id: number; /* … */ }
const short: Row[] = [];
short.push(new Row(9, "s", 9));
const missing = short[5];   // undefined
console.log(missing.id);

Reproduced against the base commit and with the representation-selection knob
off, so it is not a Ptr<Shape> defect — the guarded field-read path
itself is not throwing for an undefined receiver when the binding carries a
class annotation.

The annotation looks load-bearing: const missing: Row = short[5] makes the
declared type Named("Row"), and the field read appears to take a path that
assumes the receiver is an object rather than going through
js_object_get_field_by_name_f64's ToObject check. Worth confirming whether an
Any-typed binding throws correctly, which would localise it to the typed
fast path.

This is a silent wrong answer, not a crash, which is the worse shape: the
program continues with undefined where Node would have aborted.