engine: internals supporting the facade migration

UndoStack push_pre_executed, FolderAddChild-based true folder moves,
NodeViewDeleteCommand delete-then-reconnect support, event emission
points for the facade event bus, and related internal adjustments.

engine/undo/undostack.cpp

@@ -105,6 +105,42 @@ void UndoStack::push(UndoCommand *command, const QString &name)
 	update_actions();
 }
 
+void UndoStack::push_pre_executed(UndoCommand *command, const QString &name)
+{
+	MultiUndoCommand *mcu = dynamic_cast<MultiUndoCommand *>(command);
+	if (mcu && mcu->child_count() == 0) {
+		delete command;
+		return;
+	}
+
+	// Clear any redoable commands
+	this->beginRemoveRows(QModelIndex(), commands_.size(),
+						  commands_.size() + undone_commands_.size());
+	if (can_redo()) {
+		for (auto it = undone_commands_.cbegin(); it != undone_commands_.cend();
+			 it++) {
+			delete (*it).command;
+		}
+		undone_commands_.clear();
+	}
+	this->endRemoveRows();
+
+	// Push without redoing: the caller already executed the children.
+	this->beginInsertRows(QModelIndex(), commands_.size(), commands_.size());
+	commands_.push_back({ command, name });
+	this->endInsertRows();
+
+	// Delete oldest
+	if (commands_.size() > k_max_undo_commands) {
+		this->beginRemoveRows(QModelIndex(), 0, 0);
+		delete commands_.front().command;
+		commands_.pop_front();
+		this->endRemoveRows();
+	}
+
+	update_actions();
+}
+
 void UndoStack::jump(size_t index)
 {
 	while (commands_.size() > index) {

engine/undo/undostack.h

@@ -40,6 +40,15 @@ public:
 
 	void push(UndoCommand *command, const QString &name);
 
+  /**
+   * @brief Push a command that has already been executed (redo skipped).
+   *
+   * Used by the facade undo-group: child commands are added to the group
+   * and executed eagerly, then the whole group is pushed with this method
+   * so it is not redone again.  Empty commands are discarded.
+   */
+  void push_pre_executed(UndoCommand *command, const QString &name);
+
 	void jump(size_t index);
 
 	void clear();
@@ -78,6 +87,40 @@ public:
 	virtual bool
 	hasChildren(const QModelIndex &parent = QModelIndex()) const override;
 
+	// Facade accessors (oakengine/undo.h C ABI): row-based history queries.
+	// Rows 0..done_count()-1 are done commands (commands_ in order), rows
+	// done_count()..command_count()-1 are undone commands (undone_commands_
+	// in order, most recently undone first).
+	int command_count() const
+	{
+		return int(commands_.size() + undone_commands_.size());
+	}
+
+	int done_count() const
+	{
+		return int(commands_.size());
+	}
+
+	bool command_is_done(int row) const
+	{
+		return row >= 0 && row < done_count();
+	}
+
+	QString command_name(int row) const
+	{
+		if (row < 0 || row >= command_count()) {
+			return QString();
+		}
+		if (row < done_count()) {
+			auto it = commands_.begin();
+			std::advance(it, row);
+			return it->name;
+		}
+		auto it = undone_commands_.begin();
+		std::advance(it, row - done_count());
+		return it->name;
+	}
+
 signals:
 	void index_changed(int i);