feat: E2E encrypted Tor channel messages (ChaCha20-Poly1305)

Messages between federated nodes are now end-to-end encrypted:
- X25519 ECDH key agreement from existing ed25519 node identities
- HKDF-SHA256 key derivation with domain separation
- ChaCha20-Poly1305 authenticated encryption per message
- Random 12-byte nonce per message via OsRng (CSPRNG)
- Graceful fallback to plaintext if encryption fails
- Receiver auto-detects encrypted vs plaintext messages

The Tor transport was already encrypted (onion routing), this adds
application-layer E2E encryption so even a compromised receiving
backend can't read messages without the node's private key.

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

core/archipelago/src/api/rpc/peers.rs

@@ -89,7 +89,25 @@ impl RpcHandler {
 
         let (data, _) = self.state_manager.get_snapshot().await;
         let pubkey = data.server_info.pubkey.clone();
-        node_message::send_to_peer(onion, &pubkey, message).await?;
+
+        // Load signing key for E2E encryption
+        let identity_dir = self.config.data_dir.join("identity");
+        let node_id = crate::identity::NodeIdentity::load_or_create(&identity_dir).await?;
+
+        // Look up recipient's pubkey from federation nodes
+        let fed_nodes = federation::load_nodes(&self.config.data_dir).await.unwrap_or_default();
+        let recipient_pubkey = fed_nodes.iter()
+            .find(|n| n.onion == onion || n.onion == format!("{}.onion", onion)
+                || format!("{}.onion", n.onion) == onion)
+            .map(|n| n.pubkey.clone());
+
+        node_message::send_to_peer(
+            onion,
+            &pubkey,
+            message,
+            Some(node_id.signing_key()),
+            recipient_pubkey.as_deref(),
+        ).await?;
         Ok(serde_json::json!({ "ok": true, "sent_to": onion }))
     }