feat(canvas): Create SpatialIndex as rbush wrapper for viewport culling and hit testing

frontend/src/canvas/SpatialIndex.ts

@@ -0,0 +1,60 @@
+import RBush from 'rbush';
+
+type BBox = {
+  minX: number;
+  minY: number;
+  maxX: number;
+  maxY: number;
+  id?: string;
+};
+
+type SpatialItem = {
+  id: string;
+  minX: number;
+  minY: number;
+  maxX: number;
+  maxY: number;
+  [key: string]: any; // Allow additional properties
+};
+
+export class SpatialIndex {
+  private rbush: RBush<SpatialItem>;
+
+  constructor() {
+    this.rbush = new RBush<SpatialItem>();
+  }
+
+  public insert(item: BBox & { id: string }): void {
+    this.rbush.insert(item as SpatialItem);
+  }
+
+  public remove(item: BBox & { id: string }): void {
+    // RBush requires the exact same object for removal
+    // In practice, you might need to search for the item first
+    this.rbush.remove(item as SpatialItem);
+  }
+
+  public search(bbox: BBox): SpatialItem[] {
+    return this.rbush.search(bbox);
+  }
+
+  public load(items: (BBox & { id: string })[]): void {
+    this.rbush.load(items as SpatialItem[]);
+  }
+
+  public clear(): void {
+    this.rbush.clear();
+  }
+
+  public all(): SpatialItem[] {
+    return this.rbush.all();
+  }
+
+  public toJSON(): any {
+    return this.rbush.toJSON();
+  }
+
+  public fromJSON(data: any): void {
+    this.rbush.fromJSON(data);
+  }
+}