feat(core): add device module with ed25519 signing

OpenSSH-format keypair generation, signing, and verification.
Foundation for device authentication.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

crates/relicario-core/Cargo.toml

@@ -15,6 +15,7 @@ sha2 = "0.10"
 sha1 = "0.10"
 hmac = "0.12"
 ed25519-dalek = { version = "2", features = ["rand_core"] }
+ssh-key = { version = "0.6", features = ["ed25519", "std"] }
 image = { version = "0.25", default-features = false, features = ["jpeg"] }
 
 # Typed-item additions