SafetyNumberCodec

Machine-readable safety number payload, version 1, for a QR code or any other channel the application picks. Both devices produce identical bytes.

version:u8 = 1
| u32 length | first userId (UTF-8)  | u32 length | first deviceId (UTF-8)
| u32 length | second userId (UTF-8) | u32 length | second deviceId (UTF-8)
| fingerprint (32)

Holds no key material. Decoding is strict (version, valid UTF-8, canonical address order, exact sizes, no trailing bytes) and throws IllegalArgumentException otherwise.