Add alphabetical auto-numbering (a, b, ... z, aa, ab, ...) (#579)

Adds a real base-26 incrementing part type to the autonumbering engine,
alongside the 14 existing NumStrategy leaves. Unlike StringNum (a fixed,
non-incrementing text segment), AlphaNum::next()/previous() carry/borrow
entirely within the part's own value -- the composition loop in
NumerotationContextCommands doesn't need to change, since (unlike #578's
wrap-and-carry) nothing here needs to signal an adjacent part.

- incrementAlpha()/decrementAlpha() implement the spreadsheet-column-name
  algorithm: increment carries right-to-left on 'z'/'Z' overflow,
  prepending a new leading letter if the whole value overflows (z -> aa,
  az -> ba). decrement is the exact inverse, including the symmetric
  shrink case (aa -> z) once every position has borrowed. A single letter
  already at "a"/"A" has no representable predecessor and is clamped
  rather than turned into "z" -- caught via manual testing, since the
  initial implementation mutated the string in the borrow loop before
  checking whether to clamp, silently discarding the original value.
- Registered in NumerotationContext::validRegExpNum() but deliberately
  not in validRegExpNumber(), so addValue() doesn't force alphabetic
  values through int conversion.
- New "Cyclique"-adjacent "Alphabétique" entry in numparteditorw's type
  dropdown, with its own letters-only QRegularExpressionValidator; the
  increase spinbox is disabled since the step is always exactly one
  letter, not a configurable amount.

Also wires the new part type through to actual element/conductor labels,
which turned out to be required for the feature to do anything visible
beyond folio numbering (which applies a NumerotationContext's
represented string directly). Element and conductor numbering instead
go through a separate formula-substitution layer
(autonum::sequentialNumbers + %sequ_/%seqt_/%seqh_-style placeholders in
AssignVariables::assignSequence()) that numerotationContextToFormula()
auto-populates. Without a matching placeholder, an "alpha" part would
silently vanish from the generated formula and never reach the label,
even though the underlying counter was advancing correctly:
- sequentialNumbers gained an `alpha` QStringList member (copy ctor,
  operator=, operator==, toXml/fromXml, clear()).
- numerotationContextToFormula() emits a new %seqa_N placeholder for
  alpha parts, the same way %sequ_N is emitted for unit parts.
- setSequential()/setSequentialToList() populate seqStruct.alpha,
  passing the raw string through as-is rather than the .toInt()-based
  formatting used for the numeric part types.
- AssignVariables::assignSequence() substitutes %seqa_N from
  seqStruct.alpha, mirroring the existing %sequ_N/%seqt_N/%seqh_N
  substitutions.
No "alphafolio" variant was added, matching the discussion's scope (only
unit/ten/hundred have folio-anchored variants).

Verified against production code via the numbering config dialog's own
Suivant/Précédent buttons: from "a", 25 clicks reached "z"; one more
produced "aa"; 25 more reached "az"; one more produced "ba" (carry).
Reversed: "ba"->"az"->(25 clicks)->"aa"->"z" (shrink)->(25 clicks)->"a".
One more "previous" at "a" correctly stayed at "a" after the clamp fix.
Also confirmed the Formule field auto-updates to "%seqa_1" the instant
the type is switched to "Alphabétique", confirming the formula-generation
wiring works live in the UI, not just at the engine level.

sources/autoNum/ui/numparteditorw.cpp

@@ -18,6 +18,8 @@
 #include "numparteditorw.h"
 #include "ui_numparteditorw.h"
 
+#include <QRegularExpressionValidator>
+
 /**
 	@brief NumPartEditorW::NumPartEditorW
 	Constructor
@@ -28,6 +30,7 @@ NumPartEditorW::NumPartEditorW(int type, QWidget *parent) :
 	QWidget(parent),
 	ui(new Ui::NumPartEditorW),
 	intValidator (new QIntValidator(0,99999,this)),
+	alphaValidator (new QRegularExpressionValidator(QRegularExpression("[A-Za-z]+"), this)),
 	m_edited_type(type)
 {
 	ui -> setupUi(this);
@@ -51,6 +54,7 @@ NumPartEditorW::NumPartEditorW (NumerotationContext &context,
 	QWidget(parent),
 	ui(new Ui::NumPartEditorW),
 	intValidator (new QIntValidator(0,99999,this)),
+	alphaValidator (new QRegularExpressionValidator(QRegularExpression("[A-Za-z]+"), this)),
 	m_edited_type(type)
 {
 	ui -> setupUi(this);
@@ -73,6 +77,8 @@ NumPartEditorW::NumPartEditorW (NumerotationContext &context,
 			setType(NumPartEditorW::hundredfolio, true);
 		else if (strl.at(0)=="wrap")
 			setType(NumPartEditorW::wrap, true);
+		else if (strl.at(0)=="alpha")
+			setType(NumPartEditorW::alpha);
 		else if (strl.at(0)=="string")
 			setType(NumPartEditorW::string);
 		else if (strl.at(0)=="idfolio")
@@ -102,6 +108,7 @@ NumPartEditorW::NumPartEditorW (NumerotationContext &context,
 NumPartEditorW::~NumPartEditorW()
 {
 	delete intValidator;
+	delete alphaValidator;
 	delete ui;
 }
 
@@ -115,6 +122,7 @@ void NumPartEditorW::setVisibleItems()
 			<< tr("Chiffre 01")
 			<< tr("Chiffre 001")
 			<< tr("Cyclique (modulo)")
+			<< tr("Alphabétique")
 			<< tr("Texte");
 	}
 	else if (m_edited_type == 1)
@@ -126,6 +134,7 @@ void NumPartEditorW::setVisibleItems()
 			<< tr("Chiffre 001")
 			<< tr("Chiffre 001 - Folio")
 			<< tr("Cyclique (modulo)")
+			<< tr("Alphabétique")
 			<< tr("Texte")
 			<< tr("N° folio")
 			<< tr("Folio")
@@ -140,6 +149,7 @@ void NumPartEditorW::setVisibleItems()
 		      << tr("Chiffre 001")
 		      << tr("Chiffre 001 - Folio")
 		      << tr("Cyclique (modulo)")
+		      << tr("Alphabétique")
 		      << tr("Texte")
 		      << tr("N° folio")
 		      << tr("Folio")
@@ -205,6 +215,9 @@ NumerotationContext NumPartEditorW::toNumContext()
 		case wrap:
 			type_str = "wrap";
 			break;
+		case alpha:
+			type_str = "alpha";
+			break;
 	}
 	if (type_str == "unitfolio"
 			|| type_str == "tenfolio"
@@ -278,6 +291,8 @@ void NumPartEditorW::on_type_cb_activated(int) {
 		setType(elementprefix);
 	else if (ui->type_cb->currentText() == tr("Cyclique (modulo)"))
 		setType(wrap);
+	else if (ui->type_cb->currentText() == tr("Alphabétique"))
+		setType(alpha);
 	emit changed();
 }
 
@@ -349,6 +364,7 @@ void NumPartEditorW::setType(NumPartEditorW::type t, bool fnum) {
 	}
 	//@t isn't a numeric type
 	else if (t == string
+		 || t == alpha
 		 || t == folio
 		 || t == idfolio
 		 || t == elementline
@@ -362,6 +378,13 @@ void NumPartEditorW::setType(NumPartEditorW::type t, bool fnum) {
 			ui -> value_field -> setValidator(nullptr);
 			ui -> value_field -> setEnabled(true);
 		}
+		else if (t==alpha) {
+				//Alphabetic step is always exactly one letter (a, b, ...);
+				//there is no numeric "increase" to configure, unlike the
+				//digit-based part types.
+			ui -> value_field -> setValidator(alphaValidator);
+			ui -> value_field -> setEnabled(true);
+		}
 		else if (t==folio) {
 			ui -> value_field -> setDisabled(true);
 			ui -> increase_spinBox -> setDisabled(true);
@@ -414,6 +437,8 @@ void NumPartEditorW::setCurrentIndex(NumPartEditorW::type t) {
 		i = ui->type_cb->findText(tr("Chiffre 001"));
 	else if (t == hundredfolio)
 		i = ui->type_cb->findText(tr("Chiffre 001 - Folio"));
+	else if (t == alpha)
+		i = ui->type_cb->findText(tr("Alphabétique"));
 	else if (t == string)
 		i = ui->type_cb->findText(tr("Texte"));
 	else if (t == idfolio)