Cancel in-progress reorders when the list is torn down

[elizrus]

Summary

Fixes a crash when a list is torn down while an interactive reorder is still in progress — for example, the user navigates away (taps a nav-bar shortcut, pops the screen) while still holding a drag.

A reorder gesture drives a native UICollectionView interactive-movement session, which UIKit expects to be resolved with endInteractiveMovement() or cancelInteractiveMovement(). Today nothing ends that session when the list leaves the window or is deallocated:

• deinit only nils collectionView.delegate/dataSource.
• didMoveToWindow() does nothing when window == nil.
• The only caller of cancelAllInProgressReorders() is the content-diff-apply path (guarded by hasInProgressReorders, and annotated as experimental/"will likely crash").

So the interactive-movement session outlives the content it was started against. A later content update or layout pass then reads a now-stale index and traps:

Swift runtime failure: Index out of range
Array.subscript.getter
ListLayoutContent.item(at:)
closure #1 in ListLayoutContent.move(from:to:)   // via CollectionViewLayout.invalidateLayout(with:)

(also observed via CollectionViewLayout.positionForItem(at:) ← ListView.setPresentationStateItemPositions().)

Change

Cancel any in-progress reorder while the data source and layout are still in sync:

• In didMoveToWindow() when the list leaves the window (the navigate-away-mid-drag case).
• In deinit as a backstop.

Both are guarded by hasInProgressReorders, so they are no-ops in the common case. hasInProgressReorders is relaxed from private to internal so tests can assert on it.

Testing

Adds ListView.ReorderTeardownTests covering both teardown paths (window removal and deinit). Verified the reorder state is cleared on window exit and that deallocation with a live reorder does not crash.

Reported from Square Point of Sale: app crashes when a payment type is dragged between sections in Payment settings and a navigation-bar shortcut is tapped mid-drag.

Checklist

• Ensure any public-facing changes are reflected in the changelog. Include them in the Main section.