PMsrHighlighter: fix keyword bugs and C++17 tidy-ups

- Fix missing comma that concatenated "ab" and "skg" into the
  bogus keyword "abskg", dropping abragam/skewedGss highlighting.
- Remove duplicate "snkzf" short name in the theory keyword list.
- Add missing theory keywords: "sg" (simpleGss short form),
  "userFcn" and its short form "u", to match src/include/PTheory.h.
- Use std::as_const in highlightBlock's range-for to avoid a
  potential QVector detach on every call.
- Pass QTextCharFormat to addRule by const reference.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/musredit_qt6/musredit/PMsrHighlighter.cpp

@@ -30,6 +30,8 @@
  *   59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.             *
  ***************************************************************************/
 
+#include <utility>
+
 #include <QGuiApplication>
 #include <QStyleHints>
 #include <QMessageBox> //as35
@@ -170,12 +172,13 @@ PMsrHighlighter::PMsrHighlighter(QTextDocument *parent)
       "Bessel",       "internbsl",   "internFldGK", "internFldLL",
       "F_mu_F",       "abragam",     "skewedGss",   "staticNKZF",
       "staticNKTF",   "dynamicNKZF", "dynamicNKTF", "muMinusExpTF",
-      "polynom"};
+      "polynom",      "userFcn"};
 
   QStringList theory_keywords_short = {"c",
                                        "a",
                                        "se",
                                        "ge",
+                                       "sg",
                                        "stg",
                                        "sgktlf",
                                        "dgktlf",
@@ -196,15 +199,15 @@ PMsrHighlighter::PMsrHighlighter(QTextDocument *parent)
                                        "ifgk",
                                        "ifll",
                                        "fmuf",
-                                       "ab"
+                                       "ab",
                                        "skg",
                                        "snkzf",
                                        "snktf",
-                                       "snkzf",
                                        "dnkzf",
                                        "dnktf",
                                        "mmsetf",
-                                       "p"};
+                                       "p",
+                                       "u"};
 
   // combine both theory keyword lists
   QStringList theory_keywords = theory_keywords_long + theory_keywords_short;
@@ -457,7 +460,7 @@ PMsrHighlighter::PMsrHighlighter(QTextDocument *parent)
  * to the Qt layout interface engine.
  */
 void PMsrHighlighter::addRule(const QStringList &keywords,
-                              const QTextCharFormat formatting) {
+                              const QTextCharFormat &formatting) {
   // Instantiate the HighlighingRule struct.
   HighlightingRule rule;
 
@@ -488,7 +491,7 @@ void PMsrHighlighter::addRule(const QStringList &keywords,
 void PMsrHighlighter::highlightBlock(const QString &text) {
 
   // Sequentially process each entry stored inside the rules vector array.
-  for (const HighlightingRule &rule : highlightingRules) {
+  for (const HighlightingRule &rule : std::as_const(highlightingRules)) {
     // Execute a global search through the single block string provided.
     QRegularExpressionMatchIterator matchIterator =
         rule.pattern.globalMatch(text);

src/musredit_qt6/musredit/PMsrHighlighter.h

@@ -96,7 +96,7 @@ private:
    * @brief Helper function for passing formatting information to the
    * highlightingRules dynamic array.
    */
-  void addRule(const QStringList &keywords, const QTextCharFormat formatting);
+  void addRule(const QStringList &keywords, const QTextCharFormat &formatting);
 
   QTextCharFormat blockFormat;  ///< Formatting profile for BLOCK keywords.
   QTextCharFormat titleFormat;  ///< Formatting profile for TITLE line.