Derive the GET SERIAL response from the device UUID

[FlorianFranzen]

Every device answered the Yubico GET SERIAL extension with the same made-up constant 00 52 F7 43. That breaks clients that select a card by serial or handle several cards at once: yubikey.rs keys its card handle on the serial, and ykman lists devices by it.

Use the first four bytes of the per-device UUID that the runner already passes in through Options for the CHUID. A simple truncation rather than a hash: the serial only needs to be stable and distinct between devices, the UUID's leading bytes already are, and the mapping stays recognizable when debugging. Platforms that do not provide a UUID keep the previous constant, so existing setups (and the aa_serial test configuration) see no change.

Tested in command_response.ron for both the UUID-derived serial and the constant fallback.