ArkAPI client v0.1.7 verification

1. Pin the expected public-key fingerprint from a trusted ArkAPI announcement:
   894ad62142c0b5077ddf255f81b236afd925c6c01e0f921f732f391059f67d82
2. Verify the downloaded public key:
   openssl pkey -pubin -in signing-key.pub.pem -outform DER | sha256sum
3. Verify the detached Ed25519 signature:
   openssl pkeyutl -verify -pubin -inkey signing-key.pub.pem -rawin \
     -in SHA256SUMS -sigfile SHA256SUMS.sig
4. Verify the artifacts:
   sha256sum -c SHA256SUMS

The private signing key is never included in a release.