diag: tracing for direct-call signal loop + CallEngine::start stages

User reports tapping "answer" on an incoming direct call does nothing
visible, and suspects the same may affect desktop. The signal recv loop
had no tracing at all, so we can't tell whether CallSetup is being
received, whether the recv loop died silently, or whether
CallEngine::start is failing between "identity loaded" and
"connected to relay, handshake complete".

- register_signal recv loop now logs every message type with fields
  (CallRinging, DirectCallOffer, DirectCallAnswer, CallSetup, Hangup,
  unhandled), plus a warn! on recv errors and a final warn when the
  loop exits.
- place_call / answer_call commands log entry + success / error. The
  answer_call error path logs the underlying send_signal error so we
  can see it in logcat instead of only in the JS error toast.
- CallEngine::start android branch logs relay/room/alias on entry,
  logs "endpoint created, dialing relay" between create_endpoint and
  connect, "QUIC connection established, performing handshake" between
  connect and perform_handshake, and promotes all three potential
  failures to explicit error! logs so a silent hang / error becomes
  visible in logcat.

No functional changes — pure diagnostics. Stacks on b35a6b7 (the Oboe
stack-pointer-escape fix) so build #43 carries both.

desktop/src-tauri/src/engine.rs

@@ -110,9 +110,11 @@ impl CallEngine {
     where
         F: Fn(&str, &str) + Send + Sync + 'static,
     {
+        info!(%relay, %room, %alias, %quality, "CallEngine::start (android) invoked");
         let _ = rustls::crypto::ring::default_provider().install_default();
 
         let relay_addr: SocketAddr = relay.parse()?;
+        info!(%relay_addr, "resolved relay addr");
 
         // Identity via shared helper (uses Tauri path().app_data_dir()).
         let seed = crate::load_or_create_seed()
@@ -123,9 +125,14 @@ impl CallEngine {
 
         // QUIC transport + handshake.
         let bind_addr: SocketAddr = "0.0.0.0:0".parse().unwrap();
-        let endpoint = wzp_transport::create_endpoint(bind_addr, None)?;
+        let endpoint = wzp_transport::create_endpoint(bind_addr, None)
+            .map_err(|e| { error!("create_endpoint failed: {e}"); e })?;
+        info!("endpoint created, dialing relay");
         let client_config = wzp_transport::client_config();
-        let conn = wzp_transport::connect(&endpoint, relay_addr, &room, client_config).await?;
+        let conn = wzp_transport::connect(&endpoint, relay_addr, &room, client_config)
+            .await
+            .map_err(|e| { error!("connect failed: {e}"); e })?;
+        info!("QUIC connection established, performing handshake");
         let transport = Arc::new(wzp_transport::QuinnTransport::new(conn));
 
         let _session = wzp_client::handshake::perform_handshake(
@@ -133,7 +140,8 @@ impl CallEngine {
             &seed.0,
             Some(&alias),
         )
-        .await?;
+        .await
+        .map_err(|e| { error!("perform_handshake failed: {e}"); e })?;
         info!("connected to relay, handshake complete");
         event_cb("connected", &format!("joined room {room}"));