feat(ui): direct-only mode setting (no relay fallback)

New toggle in Settings → "Direct-only mode (no relay fallback)":
- Default: OFF (normal behavior, relay fallback on P2P failure)
- When ON: connect returns error if P2P fails, with full
  candidate_diags in the debug log showing why each candidate
  failed. Call never falls back to relay.

Useful for testing NAT traversal — you see the exact failure
reason instead of the call silently working through relay.

Wired end-to-end:
- Settings.directOnly persisted in localStorage
- Passed as directOnly param to Rust connect command
- connect:path_negotiated shows direct_only flag
- connect:direct_only_failed emits on failure with diags

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

desktop/index.html

@@ -187,6 +187,10 @@
               <input id="s-call-debug" type="checkbox" />
               Call flow debug logs (trace every step of a call)
             </label>
+            <label class="checkbox">
+              <input id="s-direct-only" type="checkbox" />
+              Direct-only mode (no relay fallback — fails if P2P can't connect)
+            </label>
           </div>
           <div class="settings-section" id="s-call-debug-section" style="display:none">
             <h3>Call Debug Log</h3>

desktop/src-tauri/src/lib.rs

@@ -333,13 +333,19 @@ async fn connect(
     // Phase 8 (Tailscale-inspired): peer's port-mapped external
     // address from NAT-PMP/PCP/UPnP, carried in CallSetup.
     peer_mapped_addr: Option<String>,
+    // Debug: when true, skip relay fallback entirely — the call
+    // fails if direct P2P doesn't connect. Useful for testing NAT
+    // traversal without the relay masking failures.
+    direct_only: Option<bool>,
 ) -> Result<String, String> {
+    let force_direct = direct_only.unwrap_or(false);
     emit_call_debug(&app, "connect:start", serde_json::json!({
         "relay": relay,
         "room": room,
         "peer_direct_addr": peer_direct_addr,
         "peer_local_addrs": peer_local_addrs,
         "peer_mapped_addr": peer_mapped_addr,
+        "direct_only": force_direct,
     }));
     let mut engine_lock = state.engine.lock().await;
     if engine_lock.is_some() {
@@ -572,7 +578,20 @@ async fn connect(
                             "local_direct_ok": local_direct_ok,
                             "peer_direct_ok": peer_direct_ok,
                             "chosen_path": format!("{:?}", chosen_path),
+                            "direct_only": force_direct,
                         }));
+
+                        // direct_only mode: refuse relay fallback
+                        if force_direct && !use_direct {
+                            let reason = format!(
+                                "direct_only: P2P failed (local_ok={local_direct_ok}, peer_ok={peer_direct_ok})"
+                            );
+                            emit_call_debug(&app, "connect:direct_only_failed", serde_json::json!({
+                                "reason": reason,
+                                "candidate_diags": race_result.candidate_diags,
+                            }));
+                            return Err(reason);
+                        }
                         tracing::info!(
                             ?chosen_path,
                             use_direct,

desktop/src/main.ts

@@ -208,6 +208,7 @@ const sAlias = document.getElementById("s-alias") as HTMLInputElement;
 const sOsAec = document.getElementById("s-os-aec") as HTMLInputElement;
 const sDredDebug = document.getElementById("s-dred-debug") as HTMLInputElement;
 const sCallDebug = document.getElementById("s-call-debug") as HTMLInputElement;
+const sDirectOnly = document.getElementById("s-direct-only") as HTMLInputElement;
 const sCallDebugSection = document.getElementById("s-call-debug-section") as HTMLDivElement;
 const sCallDebugLogEl = document.getElementById("s-call-debug-log") as HTMLDivElement;
 const sCallDebugClearBtn = document.getElementById("s-call-debug-clear") as HTMLButtonElement;
@@ -287,6 +288,9 @@ interface Settings {
   /// renders into the rolling Debug Log panel in settings. Off in
   /// normal mode keeps the GUI quiet but logcat always has a copy.
   callDebugLogs: boolean;
+  /// Debug: skip relay fallback on direct calls — fail if P2P
+  /// doesn't connect. Useful for testing NAT traversal.
+  directOnly: boolean;
 }
 
 function loadSettings(): Settings {
@@ -301,6 +305,7 @@ function loadSettings(): Settings {
     osAec: true, agc: true, quality: "auto", recentRooms: [],
     dredDebugLogs: false,
     callDebugLogs: false,
+    directOnly: false,
   };
   try {
     const raw = localStorage.getItem("wzp-settings");
@@ -1175,6 +1180,7 @@ function openSettings() {
   sRoom.value = s.room; sAlias.value = s.alias; sOsAec.checked = s.osAec;
   sDredDebug.checked = !!s.dredDebugLogs;
   sCallDebug.checked = !!s.callDebugLogs;
+  sDirectOnly.checked = !!s.directOnly;
   // Show the debug-log panel only when the user has the flag on —
   // keeps the settings panel short in normal use.
   sCallDebugSection.style.display = s.callDebugLogs ? "" : "none";
@@ -1337,6 +1343,7 @@ settingsSave.addEventListener("click", () => {
   s.quality = QUALITY_STEPS[parseInt(sQuality.value)] || "auto";
   s.dredDebugLogs = sDredDebug.checked;
   s.callDebugLogs = sCallDebug.checked;
+  s.directOnly = sDirectOnly.checked;
   saveSettingsObj(s);
   // Push the new flags to the Rust side immediately so the next
   // frame / call already honors them without waiting for a restart.
@@ -1692,6 +1699,7 @@ listen("signal-event", (event: any) => {
             peerDirectAddr: data.peer_direct_addr ?? null,
             peerLocalAddrs: data.peer_local_addrs ?? [],
             peerMappedAddr: data.peer_mapped_addr ?? null,
+            directOnly: loadSettings().directOnly || false,
           });
           showCallScreen();
         } catch (e: any) {