refactor: extend PhysicsBody with cached dims and add dirtyElements

lib/gravity-engine.ts

@@ -4,6 +4,10 @@ interface PhysicsBody {
   y: number;
   vx: number;
   vy: number;
+  width: number;
+  height: number;
+  originX: number;
+  originY: number;
   isDragging: boolean;
   startX: number;
   startY: number;
@@ -11,6 +15,7 @@ interface PhysicsBody {
 
 export class GravityEngine {
   private bodies: PhysicsBody[] = [];
+  private dirtyElements: HTMLElement[] = [];
   private animationFrameId: number | null = null;
   private isRunning = false;
 
@@ -83,6 +88,10 @@ export class GravityEngine {
         y: rect.top,
         vx: (Math.random() - 0.5) * 8, // slight explosion outward
         vy: (Math.random() - 0.5) * 5, // slight pop upward
+        width: rect.width,
+        height: rect.height,
+        originX: rect.left,
+        originY: rect.top,
         isDragging: false,
         startX: 0,
         startY: 0,