…it platforms

Compute ``final_depth`` in ``decode_stack_pop_push()`` and
``decode_stack_suffix()`` using ``uint64_t`` before validating it.

On 32-bit builds, using ``size_t`` arithmetic for ``keep + push`` can wrap
for large input values, causing the later bounds check to validate the wrong
final depth. Using a widened type keeps the validation aligned with the
actual result.