fix(assembly): update flip-detection baseline during drag steps

During drag operations, validateNewPlacements() compared each solver
result against the pre-drag positions saved once in preDrag().  As the
user dragged further, the cumulative rotation from that fixed baseline
easily exceeded the 91-degree threshold, causing valid intermediate
results to be rejected with 'flipped orientation' warnings and making
parts appear to explode.

Fix: call savePlacementsForUndo() after each accepted drag step so
that the flip check compares against the last accepted state rather
than the original pre-drag origin.

src/Mod/Assembly/App/AssemblyObject.cpp

@@ -509,6 +509,14 @@ void AssemblyObject::doDragStep()
         if (validateNewPlacements()) {
             setNewPlacements();
 
+            // Update the baseline positions after each accepted drag step so that
+            // the orientation-flip check in validateNewPlacements() compares against
+            // the last accepted state rather than the pre-drag origin.  Without this,
+            // cumulative rotation during a long drag easily exceeds the 91-degree
+            // threshold and causes the solver result to be rejected ("flipped
+            // orientation"), making parts appear to explode.
+            savePlacementsForUndo();
+
             auto joints = getJoints(false);
             for (auto* joint : joints) {
                 if (joint->Visibility.getValue()) {