Fix: positionKey() didn't sort coordinates numerically

Plain fixed-precision formatting ("%.4f") produces strings that don't
compare in numeric order once the integer part has a different digit
count -- e.g. "15.0000" sorts before "5.0000" as text, even though
15 > 5. That silently broke the determinism goal of this branch for
any diagram with coordinates spanning more than one digit width.

Shift into a non-negative range and zero-pad to a fixed width instead,
so the formatted string sorts the same way the number does, including
negative values.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

sources/diagram.cpp

@@ -45,13 +45,24 @@
 #include <math.h>
 
 namespace {
-	/// Format a position as a string that sorts the same way the numbers do
-	/// (fixed precision, so "-" and decimals compare correctly as text).
+	/// Format a coordinate as a string that sorts the same way the number
+	/// does. Plain fixed-precision formatting ("%.4f") does NOT do this --
+	/// e.g. "15.0000" sorts before "5.0000" as text even though 15 > 5 --
+	/// so shift into a non-negative range and zero-pad to a fixed width
+	/// before comparing.
+	QString coordinateKey(double v)
+	{
+			//Diagram coordinates are nowhere near this range; the offset and
+			//width just need to be big enough that shifted values are always
+			//non-negative and always the same digit count.
+		constexpr double offset = 1e9;
+		qint64 scaled = qint64(qRound64((v + offset) * 10000.0));
+		return QStringLiteral("%1").arg(scaled, 20, 10, QLatin1Char('0'));
+	}
+
 	QString positionKey(const QPointF &pos)
 	{
-		return QStringLiteral("%1|%2")
-				.arg(pos.x(), 0, 'f', 4)
-				.arg(pos.y(), 0, 'f', 4);
+		return coordinateKey(pos.x()) + QLatin1Char('|') + coordinateKey(pos.y());
 	}
 
 	/// Sort key for Diagram::toXml()'s <elements> block: the element's own