refactor: clean up verbose migration comments

Remove redundant Qt6 migration comments that restate obvious API changes:
- QtConcurrent::run parameter order explanations (preserved where lambda wrapping needed)
- QStringRef/QStringView API changes (shortened to essential info)
- QComboBox::activated signal signature change
- QMap::unite() removal semantics
- QAction include path change
- QMouseEvent/QWheelEvent position() API
- QList/QVector equivalence in Qt6
- QFuture continuation API
- Code generation comments

Preserved comments explaining non-obvious Qt6 behavior or necessary
API migration details.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

qt6-after/qt6-migrated-legatus/Lts/Application/THP/thpwidgets/thp_html_widget.cpp

@@ -32,7 +32,6 @@ public:
   bool needsToBeOpenedExternally(const QUrl &url)
   {
     const QString lpath = url.toString();
-    // Qt6: QStringRef/QString::splitRef and QString::midRef were removed; use QStringView
     const QStringView ext = QStringView{lpath}.split(QLatin1Char('.')).last();
     const QStringView ext2Chars = ext.left(2);
     if (ext == QLatin1String("html") &&