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_rubberbands.cpp

@@ -64,7 +64,6 @@ TEST(RubberbandsTest, QTouchEvent_API_Qt6)
 // Test Rubberbands QFuture continuation pattern
 TEST(RubberbandsTest, QFuture_Continuation_Pattern)
 {
-    // Qt6: QFuture supports .then() continuations
     // This pattern is used in Rubberbands for async operations
     
     QFuture<int> future = QtConcurrent::run([]() {
@@ -156,7 +155,6 @@ TEST(RubberbandsTest, Async_Operation_Simulation)
 // Test Rubberbands Qt6 container compatibility
 TEST(RubberbandsTest, Qt6_Container_Compatibility)
 {
-    // Qt6: QList and QVector are identical
     QList<QPointF> points;
     
     points.append(QPointF(0, 0));