fix(miner): close the xmrig stdout fd after the monitor join

Stopping the miner closed the stdout pipe before joining the monitor
thread, and the UI thread's stats poll also drained stdout. Either path
could read or close a descriptor the other side was still using, so a
read could land on a closed or already-reused fd.

- stop() now joins the monitor thread first and only then closes the
  process handle and the stdout pipe, so nothing is draining the pipe
  when it is closed.
- The stdout drain is serialized under its own mutex, and the UI
  thread's stats poll no longer drains stdout at all: the monitor thread
  is the sole reader. The process handle, pid and fds are atomic so they
  can be observed safely from the memory probe.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/daemon/xmrig_manager.h

@@ -177,12 +177,13 @@ private:
 
     // Process handles
 #ifdef _WIN32
-    HANDLE process_handle_ = nullptr;
-    HANDLE stdout_read_    = nullptr;
+    std::atomic<HANDLE> process_handle_{nullptr};
+    std::atomic<HANDLE> stdout_read_{nullptr};
 #else
-    pid_t process_pid_     = 0;
-    int   stdout_fd_       = -1;
+    std::atomic<pid_t> process_pid_{0};
+    std::atomic<int>   stdout_fd_{-1};
 #endif
+    std::mutex io_mutex_;   // serializes drainOutput(); lock order io_mutex_ -> output_mutex_
 
     std::thread monitor_thread_;
     std::atomic<bool> should_stop_{false};