feat: Add physics model design document, implement multi-card snapping and layout system improvements, and refine pointer event handling.

frontend/src/desktop/LayoutSystem.ts

@@ -179,6 +179,27 @@ export class LayoutCard implements LayoutCardState {
         });
     }
 
+    snapTo(x: number, y: number) {
+        this.update({ x, y });
+
+        if (!this.ref)
+            return;
+
+        this.ref.classList.add('desk-item--swoop');
+        this.ref.style.transform = `translate3d(${this.x}px, ${this.y}px, 0) rotate(${this.rotation}deg)`;
+
+        const cleanup = () => {
+            if (this.ref) {
+                this.ref.classList.remove('desk-item--swoop');
+            }
+        };
+
+        this.ref.addEventListener('transitionend', cleanup, { once: true });
+
+        // Safety timeout in case transitionend doesn't fire (e.g. element removed)
+        setTimeout(cleanup, 350);
+    }
+
     toSnapshot(): LayoutCardState {
         return {
             id: this.id,
@@ -252,14 +273,29 @@ export class LayoutStore {
         for (const card of this.items.values()) {
             const dx = card.x - x;
             const dy = card.y - y;
-            const distance = Math.sqrt(dx * dx + dy * dy);
-            if (distance < radius + card.outerRadius) {
+
+            // Check if center is within inner radius of top card
+            // Use squared distance to avoid sqrt
+            const distSq = dx * dx + dy * dy;
+            if (distSq < radius * radius) {
                 result.push(card);
             }
         }
         return result;
     }
 
+    getStackBelow(topCard: LayoutCard): string[] {
+        const candidates = this.getCardsInCircle(topCard.x, topCard.y, topCard.innerRadius);
+
+        return candidates
+            .filter(other => {
+                if (other.id === topCard.id) return false;
+                if (other.z >= topCard.z) return false;
+                return true;
+            })
+            .map(c => c.id);
+    }
+
     getSnapshot() {
         return Array.from(this.items.values()).map(card => card.toSnapshot());
     }