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/tests/test_graphical_objects.cpp

@@ -115,7 +115,6 @@ TEST(GraphicalObjectsTest, ArcGIS_Integration_Points)
 // Test GraphicalObjects container operations (Qt6 QList/QVector)
 TEST(GraphicalObjectsTest, Container_Operations)
 {
-    // Qt6: QList and QVector are identical
     QList<QPointF> points;
     points << QPointF(0, 0) << QPointF(10, 10) << QPointF(20, 20);