Fix TSAN: acquire mutex before spawning writer thread

Establish a happens-before relationship by acquiring the mutex in the
main thread before starting the writer thread. This satisfies TSAN's
requirement that mutex creation and first cross-thread access are
properly synchronized.

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

Author: facontidavide

src/loggers/bt_file_logger_v2.cpp

@@ -87,6 +87,13 @@ FileLogger2::FileLogger2(const BT::Tree& tree, std::filesystem::path const& file
   flatbuffers::WriteScalar(write_buffer.data(), timestamp_usec);
   _p->file_stream.write(write_buffer.data(), 8);
 
+  // Acquire mutex once before starting writer thread to establish
+  // happens-before relationship for TSAN (mutex created in this thread,
+  // first accessed in writer thread)
+  {
+    std::scoped_lock lock(_p->queue_mutex);
+  }
+
   _p->writer_thread = std::thread(&FileLogger2::writerLoop, this);
 
   // Wait for writer thread to signal it's ready