feat: add onItemsChange callback to PaginatedTable (#3300)

Adds an optional onItemsChange callback prop that fires whenever
the visible items change (page navigation, fetch, page size change).
Enables consumers to react to table data changes without relying
on slot prop workarounds.

Author: andrewzamojc

src/lib/holocene/table/paginated-table/api-paginated.svelte

@@ -32,6 +32,7 @@
     maxHeight?: string;
     onError?: ((error: Error | unknown) => void) | undefined;
     onFetch: () => Promise<PaginatedRequest<T>>;
+    onItemsChange?: (items: T[]) => void;
     onShiftUp?: KeyboardHandler;
     onShiftDown?: KeyboardHandler;
     onSpace?: KeyboardHandler;
@@ -51,6 +52,7 @@
   export let maxHeight = '';
   export let onError: ((error: Error) => void) | undefined = undefined;
   export let onFetch: () => Promise<PaginatedRequest<T>>;
+  export let onItemsChange: ((items: T[]) => void) | undefined = undefined;
   export let onShiftUp: KeyboardHandler = undefined;
   export let onShiftDown: KeyboardHandler = undefined;
   export let onSpace: KeyboardHandler = undefined;
@@ -182,6 +184,12 @@
     }
   }
 
+  let previousItems: T[] | undefined;
+  $: if (onItemsChange && $store.visibleItems !== previousItems) {
+    previousItems = $store.visibleItems;
+    onItemsChange($store.visibleItems);
+  }
+
   $: adjustedTotal =
     !$store.hasNext && $store.indexEnd !== total ? $store.indexEnd : total;
 </script>