Fix race condition in FileLogger2 constructor

The FileLogger2 constructor would start the writer thread after
registering status change callbacks in the base class. This created
a race condition where callbacks could fire before the writer thread
was ready, causing TSAN failures (double lock and data race errors).

Fix by adding an atomic 'ready' flag that is:
- Set to true when the writer thread starts running
- Checked in callback() before accessing the queue

Callbacks that arrive before the writer thread is ready are now
safely ignored.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: facontidavide

src/loggers/bt_file_logger_v2.cpp

@@ -27,6 +27,7 @@ struct FileLogger2::PImpl
 
   std::thread writer_thread;
   std::atomic_bool loop = true;
+  std::atomic_bool ready = false;  // Set to true when writer thread is running
 };
 
 FileLogger2::FileLogger2(const BT::Tree& tree, std::filesystem::path const& filepath)
@@ -84,6 +85,12 @@ FileLogger2::~FileLogger2()
 void FileLogger2::callback(Duration timestamp, const TreeNode& node,
                            NodeStatus /*prev_status*/, NodeStatus status)
 {
+  // Wait until the writer thread is ready to avoid race during construction
+  if(!_p->ready.load(std::memory_order_acquire))
+  {
+    return;
+  }
+
   Transition trans{};
   trans.timestamp_usec = uint64_t(ToUsec(timestamp - _p->first_timestamp));
   trans.node_uid = node.UID();
@@ -102,6 +109,9 @@ void FileLogger2::flush()
 
 void FileLogger2::writerLoop()
 {
+  // Signal that the writer thread is ready to accept callbacks
+  _p->ready.store(true, std::memory_order_release);
+
   // local buffer in this thread
   std::deque<Transition> transitions;