Function ``bit_pack_probe``: BitPackArg -> BitPackResult.
Returns ``{w, z, y, x}`` packed back into a 16-bit-wide struct that uses
the same widths as the argument but in reverse field order. Each field has
a unique width, and the chosen sentinel values used by the test driver
(``x=0b001``, ``y=0b10001``, ``z=0b1001``, ``w=0b1100``) are unique within
their own width, so a misaligned shift or wrong field offset produces a
wrong number rather than a coincidentally-matching one.
Definition at line 274 of file serialization_probes.py.