Cover auto-numbering counter changes with undo/redo

Placing an auto-numbered element or conductor advances a shared
NumerotationContext counter (QETProject::addConductorAutoNum/
addElementAutoNum) as a side effect that sat entirely outside the undo
stack. Undoing the placement removed the visible number but left the
counter advanced, so every undo of an auto-numbered placement silently
burned a number, with no way to get it back short of a manual reset.

Adds SetAutoNumContextCommand, a small QUndoCommand storing the old/new
NumerotationContext and calling the matching add*AutoNum() setter on
undo()/redo() -- the same shape QPropertyUndoCommand already uses next
to it in ConductorAutoNumerotation::applyText().

Wires it into the two conductor call sites (the static newProperties(),
and numerateNewConductor(), both in ConductorAutoNumerotation) and the
element call site (Element::setUpFormula(), called from
DiagramEventAddElement::addElement() when a new element is dropped onto
a diagram). setUpFormula() now takes an optional parent QUndoCommand;
addElement() calls it before pushing its own undo_object so the counter
change lands in the same undo macro as the element's placement -- one
Ctrl+Z reverts both together, instead of leaving the counter adrift.

The project-properties config dialog's own add*AutoNum() calls (editing
the numbering rule itself, not a side effect of placing something) are
deliberately left untouched, as are the load-time folio-sequential
bookkeeping calls in Diagram::loadElmtFolioSeq()/loadCndFolioSeq() and
the bulk folio-renumbering passes in QETProject -- none of those run as
part of an undoable user gesture.

Implements the scope proposed in discussion #608.

sources/undocommand/setautonumcontextcommand.cpp

@@ -0,0 +1,52 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#include "setautonumcontextcommand.h"
+
+#include <utility>
+
+/**
+	@brief SetAutoNumContextCommand::SetAutoNumContextCommand
+	@param setter the QETProject setter to call on undo/redo
+	(addConductorAutoNum/addElementAutoNum/addFolioAutoNum, bound to a project)
+	@param key the numerotation context's name/key
+	@param old_context the context's value before this placement
+	@param new_context the context's value after this placement
+	@param parent parent undo command
+*/
+SetAutoNumContextCommand::SetAutoNumContextCommand(
+		Setter setter,
+		const QString &key,
+		const NumerotationContext &old_context,
+		const NumerotationContext &new_context,
+		QUndoCommand *parent) :
+	QUndoCommand(parent),
+	m_setter(std::move(setter)),
+	m_key(key),
+	m_old_context(old_context),
+	m_new_context(new_context)
+{}
+
+void SetAutoNumContextCommand::redo()
+{
+	m_setter(m_key, m_new_context);
+}
+
+void SetAutoNumContextCommand::undo()
+{
+	m_setter(m_key, m_old_context);
+}

sources/undocommand/setautonumcontextcommand.h

@@ -0,0 +1,57 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#ifndef SETAUTONUMCONTEXTCOMMAND_H
+#define SETAUTONUMCONTEXTCOMMAND_H
+
+#include "../autoNum/numerotationcontext.h"
+
+#include <QUndoCommand>
+#include <functional>
+
+/**
+	@brief The SetAutoNumContextCommand class
+	Undo/redo wrapper around one of QETProject's add*AutoNum() setters
+	(conductor/element/folio numerotation counters). Placing an
+	auto-numbered item advances one of these counters as a side effect;
+	without this command the counter change sits outside the undo stack
+	entirely, so undoing the placement removes the visible number but
+	leaves the counter advanced, silently burning it.
+*/
+class SetAutoNumContextCommand : public QUndoCommand
+{
+	public:
+		using Setter = std::function<void(const QString &, const NumerotationContext &)>;
+
+		SetAutoNumContextCommand(
+				Setter setter,
+				const QString &key,
+				const NumerotationContext &old_context,
+				const NumerotationContext &new_context,
+				QUndoCommand *parent = nullptr);
+
+		void undo() override;
+		void redo() override;
+
+	private:
+		Setter m_setter;
+		QString m_key;
+		NumerotationContext m_old_context;
+		NumerotationContext m_new_context;
+};
+
+#endif // SETAUTONUMCONTEXTCOMMAND_H