A NATS nkey is an Ed25519 keypair, so the bus reuses each peer's signing
identity for the data plane instead of minting new key material. ClientNkey
derives the user nkey public string and a nonce-signing callback from the
peer's Ed25519 private key (its first 32 bytes are the nkey seed);
SignPubHexFromNkey maps a presented nkey back to the allowlist's hex key;
NkeyPublicFromSignPub is the public-only derivation.
This is NATS-specific transport glue kept in the app, not promoted to the
registry, to avoid pulling nats-io/nkeys into the multi-domain registry
module. The dedicated round-trip test runs first (spec requirement): it
proves the nkey signature equals the identity's raw Ed25519 signature and
that the nkey maps back to the identity's hex.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
