fix(assembly): use short-arc angle in validateNewPlacements (#338)

Rotation::evaluateVector() computes angle = 2*acos(w) which gives
values in [0, 2*pi].  When the relative quaternion has w < 0 (opposite
hemisphere), the angle exceeds pi even though q and -q represent the
same rotation.  This caused the validator to report ~350 degree 'flips'
and reject valid solver output.

Fix: map the angle to [0, pi] before comparing against the 91-degree
threshold.  This is the short-arc equivalent — the minimum rotation
angle between two orientations regardless of quaternion sign convention.

src/Mod/Assembly/App/AssemblyObject.cpp

@@ -659,19 +659,26 @@ bool AssemblyObject::validateNewPlacements()
             // Get the relative rotation: how much did the part rotate?
             Base::Rotation relativeRot = newRot * oldRot.inverse();
 
-            // Get the angle of this rotation
+            // Ensure we measure the short-arc angle.  Rotation stores
+            // angle = 2*acos(w) which gives [0, 2*pi].  When the
+            // quaternion w component is negative (opposite hemisphere),
+            // the angle exceeds pi even though q and -q represent the
+            // same rotation.  Map to [0, pi] for a correct comparison.
             Base::Vector3d axis;
             double angle;
             relativeRot.getRawValue(axis, angle);
+            if (angle > M_PI) {
+                angle = 2.0 * M_PI - angle;
+            }
 
             // If the part rotated more than 90 degrees, consider it a flip
             // Use 91 degrees to allow for small numerical errors
             constexpr double maxAngle = 91.0 * M_PI / 180.0;
-            if (std::abs(angle) > maxAngle) {
+            if (angle > maxAngle) {
                 Base::Console().warning(
                     "Assembly : Ignoring bad solve, part (%s) flipped orientation (%.1f degrees).\n",
                     obj->getFullLabel(),
-                    std::abs(angle) * 180.0 / M_PI
+                    angle * 180.0 / M_PI
                 );
                 return false;
             }