feat: DID persistence + federation node names in sync

Part 1 — DID Persistence:
- Deploy script creates /var/lib/archipelago/identity/ directory
- First-boot script creates identity dir with proper ownership
- Identity load now logs pubkey to confirm persistence across restarts

Part 2 — Node Names:
- NodeStateSnapshot includes node_name field
- build_local_state() passes server name to sync responses
- update_node_state() stores peer's announced name on the FederatedNode
- Names propagate automatically during federation.sync-state

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/identity.rs

@@ -35,7 +35,10 @@ impl NodeIdentity {
             let arr: [u8; 32] = bytes
                 .try_into()
                 .map_err(|_| anyhow::anyhow!("Invalid node key length"))?;
-            SigningKey::from_bytes(&arr)
+            let key = SigningKey::from_bytes(&arr);
+            let pubkey_hex = hex::encode(key.verifying_key().as_bytes());
+            tracing::info!("Loaded existing node identity (pubkey: {}...)", &pubkey_hex[..16]);
+            key
         } else {
             let signing_key = SigningKey::generate(&mut OsRng);
             fs::write(&key_path, signing_key.to_bytes())