Give each copy of a symbol sharing a uuid its own uuid when a project opens

Older versions' copy and paste could leave two symbols with the same
uuid. Wires, links and the project database name a symbol by its uuid,
so a copy was ambiguous: #1417 worked around it for wires, but the
project database keeps one row per uuid, and a link to the uuid could
mean either copy.

When a project opens, every symbol carrying a uuid another symbol also
carries gets a uuid of its own, derived from the shared one and its
folio's uuid, so the same file gives the same result on every open. The
copy that keeps the shared uuid is the first one linked to another
symbol (its partner's link names that uuid), else the first. It is done
once the folio's wires are connected, so #1417's choice of which copy a
wire meets is kept. Folios holding such a copy are built when the
project opens, also with lazy folios.

Only files with shared uuids change, once: 3 of the 341 corpus projects.

tst_sharedsymboluuid: the lamps given one uuid now come back with three;
a linked contact and an unlinked symbol sharing its uuid keep every link
and wire, the linked one keeping the uuid. Both fail without the fix.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XCdg2Er2vYhB8Vwa5G7fSR

tests/qttest/tst_sharedsymboluuid.cpp

@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
 #include <QtTest>
+#include <QSet>
 
 #include <QDomDocument>
 #include <QFile>
@@ -150,13 +151,66 @@ private slots:
 		QVERIFY(!twice.isEmpty());
 		QCOMPARE(nets(twice), expected);
 
-			//No wire names a lamp by the uuid they share
-		for (const QDomElement &c : childElements(firstDiagram(load(twice)),
-											 QStringLiteral("conductors"),
-											 QStringLiteral("conductor"))) {
-			QVERIFY(c.attribute(QStringLiteral("element1")) != shared);
-			QVERIFY(c.attribute(QStringLiteral("element2")) != shared);
+			//Each lamp has a uuid of its own after a save: one keeps the
+			//shared uuid, the others get one derived from it
+		QSet<QString> uuids;
+		for (const QDomElement &e : lamps(load(twice)))
+			uuids.insert(e.attribute(QStringLiteral("uuid")));
+		QCOMPARE(uuids.size(), 3);
+		QVERIFY(uuids.contains(shared));
+	}
+
+	// A linked contact and an unlinked symbol carrying the same uuid (a copy
+	// made by an older version): the linked one keeps the uuid, the other
+	// gets its own, and every link and wire stays where it was.
+	void linkedCopyKeepsUuid()
+	{
+		const QString source = QStringLiteral(QET_EXAMPLES_DIR "/affuteuse_250h.qet");
+		QDomDocument doc = load(source);
+		QList<QDomElement> all;
+		const QDomNodeList folios = doc.documentElement().elementsByTagName(QStringLiteral("diagram"));
+		for (int i = 0; i < folios.size(); ++i)
+			all << childElements(folios.at(i).toElement(), QStringLiteral("elements"),
+								 QStringLiteral("element"));
+		auto linked = [](const QDomElement &e) {
+			return !e.firstChildElement(QStringLiteral("links_uuids"))
+					.firstChildElement(QStringLiteral("link_uuid")).isNull();
+		};
+		QDomElement keeper, copy;
+		for (const QDomElement &e : std::as_const(all)) {
+			if (keeper.isNull() && linked(e)) keeper = e;
+			else if (copy.isNull() && !linked(e)) copy = e;
 		}
+		QVERIFY(!keeper.isNull() && !copy.isNull());
+		const QString shared = keeper.attribute(QStringLiteral("uuid"));
+		copy.setAttribute(QStringLiteral("uuid"), shared);
+		const int wires = doc.elementsByTagName(QStringLiteral("conductor")).size();
+
+		const QString saved = resave(write(doc));
+		QVERIFY(!saved.isEmpty());
+		const QDomDocument out = load(saved);
+		QHash<QString, QDomElement> by_uuid;
+		QList<std::pair<QString, QString>> links;
+		const QDomNodeList saved_folios = out.documentElement().elementsByTagName(QStringLiteral("diagram"));
+		for (int i = 0; i < saved_folios.size(); ++i)
+			for (const QDomElement &e : childElements(saved_folios.at(i).toElement(),
+													  QStringLiteral("elements"),
+													  QStringLiteral("element"))) {
+				const QString u = e.attribute(QStringLiteral("uuid"));
+				QVERIFY2(!by_uuid.contains(u), qPrintable(QStringLiteral("uuid still shared: ") + u));
+				by_uuid.insert(u, e);
+				for (QDomElement l = e.firstChildElement(QStringLiteral("links_uuids"))
+						 .firstChildElement(QStringLiteral("link_uuid"));
+					 !l.isNull(); l = l.nextSiblingElement(QStringLiteral("link_uuid")))
+					links.append({u, l.attribute(QStringLiteral("uuid"))});
+			}
+		QVERIFY(by_uuid.contains(shared));
+		QVERIFY(linked(by_uuid.value(shared)));
+		for (const auto &link : std::as_const(links)) {
+			QVERIFY2(by_uuid.contains(link.second), "a link names a symbol not in the project");
+			QVERIFY2(links.contains(std::make_pair(link.second, link.first)), "a link only one side has");
+		}
+		QCOMPARE(out.elementsByTagName(QStringLiteral("conductor")).size(), wires);
 	}
 
 	void savedByUuidReopensRight()