Describe the bug
In the Svelte Virtualizer, if scrollRef is assigned after onMount (e.g. via bind:ref from external components), the scroll container is not observed. This is because observation only happens once during onMount.

To Reproduce
Pass scrollRef to Virtualizer using bind:ref
Ref is set after onMount
Scroll container is not observed

Expected behavior
scrollRef should be observed even if it's set after onMount. Changing from onMount to $effect could ensure proper observation. Assuming that multiple calls to $observe are safe, this would be a straightforward fix.

Platform:

• OS: Windows
• Browser: Chrome
• Version of this package: 0.41.3
• Version of framework: Svelte 5+

Additional context

Let me know if a PR would be helpful—I'd be glad to make one.