DiagramPosition: compile the letter filter once

The constructor built QRegularExpression("[^A-Z]") on every call, and
compiling it cost more than the rest of Diagram::convertPosition() put
together -- about 95k instructions a call under callgrind. Every element
row of the project database pays it on each rebuild, as does anything
else that turns a scene point into a folio cell.

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

sources/diagramposition.cpp

@@ -29,7 +29,11 @@
 DiagramPosition::DiagramPosition(const QString &letter, unsigned int number) {
 	// purifie les lettres
 	letter_ = letter.toUpper();
-	letter_.remove(QRegularExpression("[^A-Z]"));
+		//Compiled once: building it here cost more than the rest of the
+		//position conversion together, once per element and per drawing item
+		//on every rebuild of the project database.
+	static const QRegularExpression not_a_letter(QStringLiteral("[^A-Z]"));
+	letter_.remove(not_a_letter);
 	number_ = number;
 }