Never derive a uuid the file already carries

A symbol keeps its derived uuid once saved, even when it is moved. A
symbol saved without a uuid that later turns up on the spot it left -- a
hand edit, an older version, another tool writing the file -- derived the
same uuid, and the project had two symbols with one identity.

readDiagramsXml() now collects every symbol and wire uuid the file
carries, on any folio, before a folio loads; derivedItemUuid() moves to
the next counter value while a candidate is among them. The result still
depends on the file alone. Renamed from derivedUuid(), which QETProject
already has for the project's own uuid.

tst_derivedsymboluuid: newcomerOnAMovedSymbolsSpotGetsAnotherUuid fails
with the check switched off.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/diagram.cpp

@@ -1685,7 +1685,7 @@ bool Diagram::fromXml(QDomElement &document,
 				//uuids anyway.
 			if (consider_informations && m_project
 				&& QUuid(element_xml.attribute(QStringLiteral("uuid"))).isNull()) {
-				nvel_elmt->setUuid(m_project->derivedUuid(
+				nvel_elmt->setUuid(m_project->derivedItemUuid(
 									   QStringLiteral("element"),
 									   QStringList{type_id,
 												   element_xml.attribute(QStringLiteral("x")),