Function ``pack_probe``: PackStruct -> PackStruct.
XORs each field with a unique sentinel of the same width. Sentinels
(``0xA0``, ``0xB000``, ``0xC0``, ``0xD0000000``) make every byte of the
reply unique and per-field-distinguishable, so a host that swaps fields
during packing produces a visibly wrong reply rather than a same-bit-width
false positive. Catches struct-field order and inter-field padding bugs;
CIRCT structs are LSB-first on the wire.
Definition at line 223 of file serialization_probes.py.