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_mapimpl.cpp

@@ -71,7 +71,7 @@ TEST(MapImplTest, Qt6_API_Compatibility)
     EXPECT_EQ(properties[QStringLiteral("zoom")].toInt(), 10);
     EXPECT_EQ(properties[QStringLiteral("center")].toPointF().x(), 45.5);
     
-    // Qt6: Replace unite() with iteration
+    // QMap::unite() removed in Qt6
     QVariantMap properties2;
     for (auto it = properties.begin(); it != properties.end(); ++it)
     {
@@ -84,7 +84,6 @@ TEST(MapImplTest, Qt6_API_Compatibility)
 // Test MapImpl container operations
 TEST(MapImplTest, Container_Operations)
 {
-    // Qt6: QList and QVector are identical
     QList<QString> layerNames;
     layerNames << QStringLiteral("BaseMap") 
                << QStringLiteral("Overlay1")