refactor(StatusChangeLogger): simplify to deferred subscription only

Remove atomic flag approach to avoid ABI break. The deferred subscription
pattern (StatusChangeLogger() + subscribeToTreeChanges()) achieves the
same thread safety without adding new members to the base class.

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

Author: facontidavide

include/behaviortree_cpp/loggers/abstract_logger.h

@@ -15,10 +15,7 @@ enum class TimestampType
 class StatusChangeLogger
 {
 public:
-  /**
-   * @brief Construct and immediately subscribe to status changes.
-   * Use this for simple loggers that don't need deferred subscription.
-   */
+  /// Construct and immediately subscribe to status changes.
   StatusChangeLogger(TreeNode* root_node);
 
   virtual ~StatusChangeLogger() = default;
@@ -61,22 +58,10 @@ class StatusChangeLogger
   }
 
 protected:
-  /**
-   * @brief Default constructor for deferred subscription mode.
-   *
-   * Use this when the derived class needs to complete initialization
-   * (e.g., start worker threads) before receiving callbacks.
-   * Call subscribeToTreeChanges() when ready to receive callbacks.
-   */
+  /// Default constructor for deferred subscription. Call subscribeToTreeChanges() when ready.
   StatusChangeLogger() = default;
 
-  /**
-   * @brief Subscribe to status changes on the tree.
-   *
-   * For loggers with threading or complex initialization, call this
-   * at the END of your constructor after all setup is complete.
-   * This ensures callbacks won't fire until you're ready to handle them.
-   */
+  /// Subscribe to status changes. Call at end of constructor for deferred subscription.
   void subscribeToTreeChanges(TreeNode* root_node);
 
 private:

src/loggers/bt_file_logger_v2.cpp

@@ -40,7 +40,7 @@ struct FileLogger2::Pimpl
 };
 
 FileLogger2::FileLogger2(const BT::Tree& tree, std::filesystem::path const& filepath)
-  : StatusChangeLogger()  // Deferred subscription - don't subscribe yet
+  : StatusChangeLogger()  // Deferred subscription
   , _p(std::make_unique<Pimpl>())
 {
   if(filepath.filename().extension() != ".btlog")
@@ -81,11 +81,7 @@ 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);
 
-  // Start the writer thread
   _p->writer_thread = std::thread(&FileLogger2::writerLoop, this);
-
-  // NOW subscribe to tree changes - after everything is initialized and
-  // the writer thread is running. This ensures callbacks are safe to handle.
   subscribeToTreeChanges(tree.rootNode());
 }
 

src/loggers/bt_sqlite_logger.cpp

@@ -57,7 +57,7 @@ void execStatement(sqlite3_stmt* stmt)
 
 SqliteLogger::SqliteLogger(const Tree& tree, std::filesystem::path const& filepath,
                            bool append)
-  : StatusChangeLogger()  // Deferred subscription - don't subscribe yet
+  : StatusChangeLogger()  // Deferred subscription
 {
   const auto extension = filepath.filename().extension();
   if(extension != ".db3" && extension != ".btdb")
@@ -129,9 +129,6 @@ SqliteLogger::SqliteLogger(const Tree& tree, std::filesystem::path const& filepa
   }
 
   writer_thread_ = std::thread(&SqliteLogger::writerLoop, this);
-
-  // NOW subscribe to tree changes - after everything is initialized and
-  // the writer thread is running. This ensures callbacks are safe to handle.
   subscribeToTreeChanges(tree.rootNode());
 }