[iOS] CarouselViewHandler2 ArgumentOutOfRangeException from deferred realization during page teardown (not fixed by #26868)

### Description

CV2 (`CarouselViewHandler2`) crashes with `System.ArgumentOutOfRangeException` (`IndexMustBeLess`) when a page hosting a `CarouselView` is being torn down (navigating away) while CV2's **deferred realization** is still pending. This was previously discussed in #26171, which was closed as a duplicate of #26863 — but we still see this crash on MAUI **10.0.51–10.0.80**, which already contain the #26863 fix (#26868), so it appears to be a distinct bug.

Crash paths (production stack traces, iOS-only):

- `CarouselViewHandler2.MapPosition` / `MapCurrentItem` → `UpdateInitialPosition` → `SetPosition` → `ScrollToItem` executing inside an `NSAsyncActionDispatcher` continuation after teardown has begun
- `GetCell` → `DetermineCellReuseId` → `ListSource.get_Item`

Both read a stale carousel index during teardown. App-side synchronous guards cannot close it: a managed predicate cannot drain an already-applied `Position` write whose deferred native continuation fires later.

**Workaround:** pinning the affected `CarouselView` to the legacy CV1 handler (`Microsoft.Maui.Controls.Handlers.Items.CarouselViewHandler`) fully eliminates the crash, localizing the bug to CV2's deferred-realization path.

We checked the CV2 fixes merged since (#35532, #35848, #32141, #34013, #36681); none targets this teardown-time path.

### Steps to Reproduce

Not deterministically reproducible locally — it is a timing race between the deferred native continuation and page teardown (multiple simulator repro attempts failed). Real-world trigger in our app: a page with a `CarouselView` (ItemsSource of a few items) is navigated away from (push to a follow-up page) shortly after the carousel appears. Full production stack traces available on request.

### Link to public reproduction project repository

_No response_

### Version with bug

10.0.80

### Is this a regression from previous behavior?

Yes, this used to work in .NET 8 (CV1 handler)

### Last version that worked well

Unknown/Other

### Affected platforms

iOS

### Affected platform versions

iOS 15.8 – 26.x

### Did you find any workaround?

Pin the affected CarouselView to the legacy CV1 handler: `handlers.AddHandler()`

### Relevant log output

```
System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection. (IndexMustBeLess)
at Microsoft.Maui.Controls.Handlers.Items2.CarouselViewController2 ... ScrollToItem (via UpdateInitialPosition/SetPosition, NSAsyncActionDispatcher continuation)
at ... GetCell → DetermineCellReuseId → ListSource.get_Item
```

Start with CarouselViewHandler2.MapPosition and MapCurrentItem, then trace UpdateInitialPosition, SetPosition, ScrollToItem, GetCell, and DetermineCellReuseId through the deferred native continuation and page teardown lifecycle. Reproduce or instrument navigation away from a page with a CarouselView, and consider the work done when the stale index no longer reaches these paths and the teardown scenario no longer throws.

Written by the indexing model from the issue text.