Give symbols saved without a uuid the same one on every load

A symbol saved without a uuid got a random one from Element::fromXml()
on every load, and the next save wrote it out: two loads of the same file
gave the same symbol two identities, and anything pointing at it by uuid
(a script, a comparison of two versions, a wire's identity) could not
follow it from one session to the next.

When a folio is loaded, such a symbol now gets a UUID v5 derived from
what it is and where it sits: its type, its position on the folio and its
orientation. Never the folio's index, so inserting or moving a folio does
not change it. Identical symbols stacked on one spot, or a copied folio,
are told apart by a counter kept per project (QETProject::derivedUuid()),
in load order among those symbols alone. A paste still renews uuids.

Symbols that have a uuid in the file keep it. All 24 example projects
already have one for every symbol, so they are unchanged; with the
symbols' uuids stripped, each saves byte-for-byte the same twice (master:
different every time).

tst_derivedsymboluuid runs --resave on a fixture with its uuids stripped:
same uuids on every load, same after a folio is inserted in front, saved
uuids kept, stacked copies differ. The first two fail without this change.

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

sources/qetproject.cpp

@@ -275,6 +275,27 @@ QUuid QETProject::uuid() const
 	return m_uuid;
 }
 
+/**
+	@brief QETProject::derivedUuid
+	A uuid for an item of this project that was saved without one, the same
+	on every load of the same file.
+	@p key describes the item by what it is, never by its place in the file
+	or its folio's index: inserting or moving a folio must not change it.
+	Items with the same @p kind and @p key anywhere in the project (a copied
+	folio, two identical symbols stacked on one spot) are told apart by a
+	counter, in load order among those items alone.
+	@return a UUID v5, which cannot collide with the v4 uuids given to new
+	items
+*/
+QUuid QETProject::derivedUuid(const QString &kind, const QString &key)
+{
+	static const QUuid derived_ns(QStringLiteral("{7d1e9c3a-5b2f-4e8a-9c61-2f4b8d0e6a17}"));
+	const QString full = kind + QLatin1Char('\n') + key;
+	const int n = m_derived_uuid_keys[full]++;
+	return QUuid::createUuidV5(derived_ns,
+							   n ? full + QLatin1Char('\n') + QString::number(n) : full);
+}
+
 /**
 	@brief QETProject::init
 */