fix(ui): Perfect symmetry for implosion animation by removing opacity tricks

Author: fabiocarneiro

lib/flutter_circle_pack_chart_painter.dart

@@ -80,25 +80,16 @@ class FlutterCirclePackChartPainter extends CustomPainter {
   void _drawImplodingNode(Canvas canvas, PackedNode node, Color parentColor) {
     final Color color = node.node.color ?? parentColor;
 
-    // Staggered threshold: children merge and stay solid until 85% of animation.
-    // This prevents the parent circle from showing through while they are still separate.
-    const double threshold = 0.85;
-    
-    final double childOpacity = (1.0 - (animationValue - threshold) / (1.0 - threshold)).clamp(0.0, 1.0);
-    final double parentOpacity = ((animationValue - threshold) / (1.0 - threshold)).clamp(0.0, 1.0);
-
+    // Exact inverse of the explosion:
+    // Children move from their packed positions back to the parent's center
+    // and grow to match the parent's radius.
     for (final child in node.children) {
-      // Move children from their packed positions back to parent center (node.x, node.y)
       final double x = child.x + (node.x - child.x) * animationValue;
       final double y = child.y + (node.y - child.y) * animationValue;
-      // Grow from child.r to parent.r (node.r)
       final double r = lerpDouble(child.r, node.r, animationValue)!;
 
-      _drawLeaf(canvas, x, y, r, child.node, color, opacity: childOpacity);
+      _drawLeaf(canvas, x, y, r, child.node, color, opacity: 1.0);
     }
-    
-    // Parent circle fades in only at the very end of the implosion
-    _drawLeaf(canvas, node.x, node.y, node.r, node.node, color, opacity: parentOpacity);
   }
 
   void _drawLeaf(