fix(migtd): avoid sending uninitialized ExchangeInformation padding

[kali834x]

ExchangeInformation carries the negotiated version range and the migration session key, and the source and destination MigTDs exchange it over the RA-TLS channel by writing the struct's raw bytes through as_bytes(). The struct had no repr(C), so the layout was the compiler's choice, and with the two u16 version fields in front of the 8-byte-aligned key there are four bytes of trailing padding after max_ver. as_bytes() copies the whole struct including that padding, and nothing writes those bytes when the value is built, so each side hands the peer four uninitialized bytes of its own memory during the key exchange. I noticed it while checking why this struct alone among the wire types in this module has no repr(C). Adding repr(C) and turning the implicit padding into an explicit zeroed reserved field closes that, and the field order keeps key, min_ver and max_ver at the offsets the compiler was already giving them, so the on-wire bytes are unchanged for peers running the current code.