Give wires saved without a uuid a lasting one, from what they connect

A wire saved without a uuid got a random one on every load, never saved
(#754): it had no identity from one session to the next, so a script
could only name it as "the wire on terminal N of symbol X", and a
comparison of two versions could not tell a moved wire from a new one.

When a folio is loaded, such a wire now gets a UUID v5 derived from its
two ends -- the symbol and terminal at each, sorted so the direction it
was drawn in does not matter -- and it is written on save. Never its
place in the file or its folio's index: inserting or moving a folio, or
saving the wires in another order, does not change it. Once saved the
uuid no longer depends on the ends, so re-connecting the wire keeps it;
QETProject::derivedItemUuid() never hands out a uuid the file already
carries, so a wire later drawn on the ends it left gets another one.
Wires that have a uuid keep it; a paste still renews them.

The 24 example projects: 3,189 wires, none with a uuid before, all 3,189
after one save, none lost, no uuid used twice in any project; two saves
of the same file are identical, and a second save keeps every wire's
uuid. Discussion #1103 has the measurements behind the recipe.

tst_derivedwireuuid runs --resave on a fixture naming ends by uuid and on
examples/tremie_vibrante.qet (ends by terminal number): every wire gets a
distinct uuid, the same on every load, read back after a save, kept per
wire when a folio is inserted, the wires are reordered or a wire is drawn
the other way, and a newcomer on a re-connected wire's old ends gets
another uuid. Without this change 14 of the 18 fail; with the uuid taken
from folio index and file order instead, the folio-insert and reorder
tests fail.

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

sources/diagram.cpp

@@ -1825,6 +1825,28 @@ bool Diagram::fromXml(QDomElement &document,
 			{
 				addItem(c);
 				c -> fromXml(f);
+					//A wire saved without a uuid got a random one that was
+					//never saved (#754), so it had no identity from one
+					//session to the next. Derive it from what it connects:
+					//the symbol and terminal at each end, sorted so the
+					//direction it was drawn in does not matter. Never its
+					//place in the file or its folio's index, so inserting or
+					//moving a folio, or saving the wires in another order,
+					//does not change it. It is saved from now on, so
+					//re-connecting the wire later keeps it; derivedItemUuid()
+					//never hands out a uuid the file already carries, so a
+					//wire later drawn on the ends it left gets another one.
+				if (consider_informations && m_project
+					&& QUuid(f.attribute(QStringLiteral("uuid"))).isNull()) {
+					auto end = [](const Terminal *t) {
+						return t->parentElement()->uuid().toString()
+								+ QLatin1Char('/') + t->stableUuid().toString();
+					};
+					QStringList ends{end(p1), end(p2)};
+					ends.sort();
+					c->setUuid(m_project->derivedItemUuid(QStringLiteral("conductor"),
+													  ends.join(QLatin1Char('\n'))));
+				}
 				added_conductors << c;
 			}
 			else