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/numerotationcontextcommands.h

@@ -144,6 +144,23 @@ class StringNum: public NumStrategy
 	NumerotationContext previous (const NumerotationContext &, const int) const override;
 };
 
+/**
+	@brief The AlphaNum class
+	Alphabetic auto-numbering (a, b, ... z, aa, ab, ...). Unlike StringNum
+	(a fixed, non-incrementing text segment), this is a real base-26
+	counter: next()/previous() carry/borrow entirely within this part's
+	own value, the same self-contained shape every other incrementing
+	NumStrategy already has.
+*/
+class AlphaNum: public NumStrategy
+{
+	public:
+	AlphaNum (Diagram *);
+	QString toRepresentedString(const QString) const override;
+	NumerotationContext next     (const NumerotationContext &, const int) const override;
+	NumerotationContext previous (const NumerotationContext &, const int) const override;
+};
+
 class IdFolioNum: public NumStrategy
 {
 	public: