Fix ListBox crash when scrolling after item removal

[NathanDrake2406]

What does the pull request do?

Prevents VirtualizingStackPanel from arranging realized containers at a NaN position while ScrollIntoView is waiting for a viewport update. This fixes the deterministic ListBox crash reported in #20085.

What is the current behavior?

After scrolling a virtualized ListBox to its end, removing an item before the realized range marks that range's stored start position as unstable. Calling ScrollIntoView(0) then intentionally defers measure until the effective viewport changes, but the intervening arrange consumes the unstable NaN position and throws InvalidOperationException: Invalid Arrange rectangle.

What is the updated/expected behavior with this PR?

The transitional arrange uses the panel's existing element-position estimator when the realized range has an unstable start position. Once the viewport update arrives, the normal measure pass replaces the estimate with the exact position. The list can scroll back to the new first item without throwing.

Validation on macOS ARM64 with .NET 10:

• Added Removing_First_Item_After_Scrolling_To_End_Should_Allow_Scrolling_To_Start, which reproduced the exception before the fix and verifies the new first item after scrolling.
• Ran ListBoxVirtualizationIssueTests: 6 passed, 0 failed.
• Ran VirtualizingStackPanelTests: 130 passed, 0 failed.
• Ran the full Avalonia.Controls.UnitTests executable: 3,648 passed, 1 existing intentional skip, 0 failed.

How was the solution implemented (if it's not obvious)?

RealizedStackElements.StartU deliberately returns NaN when collection changes make the cached position unreliable. ArrangeOverride now recognizes that sentinel and obtains a finite transitional position through GetOrEstimateElementU, the same estimator already used when positioning unrealized and focused elements.

The commits follow the repository's bug-fix convention: the first commit adds the failing behavioral test, and the second commit contains the fix.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes? No public API was added or changed.
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation. No documentation change is needed for an internal layout fix.

Breaking changes

None. The change only replaces an invalid transitional arrange coordinate with the panel's existing estimate.

Obsoletions / Deprecations

None.

Fixed issues

Fixes #20085

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0067667-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

LGTM!