netcode-revival: fix SetRelay deadlock + grant multiplayer privileges

- Deadlock: SetRelay held the transport mutex while join()ing the receive
  thread, which needs the mutex to observe running_ and exit. With the 200ms
  recv timeout the thread woke and hit the lock -> a room/server change froze
  the game (every room keystroke reconnects the transport). StopThread now
  detaches the thread under the lock and joins WITHOUT it.
- XamUserCheckPrivilege denied everything, which is the gate the game's
  'create a party' flow checks (multiplayer sessions); grant requested
  privileges so the P2P session flow opens, and log the mask.

include/rex/system/xnet_transport.h

@@ -138,7 +138,10 @@ class XNetTransport {
   XNetTransport& operator=(const XNetTransport&) = delete;
 
   void Reconnect();
-  void StopLocked();
+  // Stop the receive thread and join it WITHOUT holding mu_ (the Run thread
+  // needs mu_ to observe running_ and exit; joining under the lock deadlocks
+  // once recvfrom times out and the thread tries to lock mu_).
+  void StopThread();
   void Run();
   void HandlePacket(const uint8_t* data, int size);
 

src/kernel/xam/xam_user.cpp

@@ -430,8 +430,13 @@ u32 XamUserCheckPrivilege_entry(u32 user_index, u32 mask, mapped_u32 out_value)
     }
   }
 
-  // If we deny everything, games should hopefully not try to do stuff.
+  // netcode-revival: the game gates its party/session flow on
-  *out_value = 0;
+  // XamUserCheckPrivilege (e.g. MULTIPLAYER_SESSIONS); denying everything
+  // (the old default) made "create a party" refuse. Grant requested online
+  // privileges so the P2P session flow opens.
+  *out_value = 1;
+  REXLOG_INFO("xam: XamUserCheckPrivilege(user={}, mask=0x{:08X}) -> granted",
+              user_index, mask);
   return X_ERROR_SUCCESS;
 }
 

src/system/xnet_transport.cpp

@@ -118,8 +118,16 @@ XNetTransport& XNetTransport::Instance() {
 }
 
 XNetTransport::~XNetTransport() {
+  StopThread();  // join outside the lock (see StopThread)
   std::lock_guard<std::mutex> lock(mu_);
-  StopLocked();
+  if (sock_ != -1) {
+    SocketClose(sock_);
+    sock_ = -1;
+  }
+  active_ = false;
+  joined_ = false;
+  sessions_.clear();
+  sockets_.clear();
 }
 
 uint32_t XNetTransport::LocalVirtualIp() {
@@ -156,17 +164,29 @@ void XNetTransport::SetRelay(const char* host, uint16_t port, const char* room)
     host_ = h;
     port_ = port;
     room_ = r;
-    StopLocked();
   }
+  StopThread();  // join the old thread OUTSIDE the lock
   Reconnect();
 }
 
-void XNetTransport::StopLocked() {
+void XNetTransport::StopThread() {
-  if (thread_.joinable()) {
+  std::thread old;
+  {
+    std::lock_guard<std::mutex> lock(mu_);
     running_ = false;
-    thread_.join();
+    old = std::move(thread_);
     thread_ = {};
   }
+  if (old.joinable()) {
+    old.join();
+  }
+}
+
+void XNetTransport::Reconnect() {
+  std::lock_guard<std::mutex> lock(mu_);
+  if (host_.empty() || port_ == 0) {
+    return;
+  }
   if (sock_ != -1) {
     SocketClose(sock_);
     sock_ = -1;
@@ -174,13 +194,7 @@ void XNetTransport::StopLocked() {
   active_ = false;
   joined_ = false;
   sessions_.clear();
-}
+  relay_resolved_ = false;
-
-void XNetTransport::Reconnect() {
-  std::lock_guard<std::mutex> lock(mu_);
-  if (host_.empty() || port_ == 0) {
-    return;
-  }
   sockaddr_in relay_addr = {};
   if (!Resolve(host_, port_, &relay_addr)) {
     REXLOG_WARN("xnet transport: cannot resolve relay '{}:{}'", host_, port_);