Fix iOS maintainVisibleContentPosition when the anchor straddling the top edge is resized

[mozzius]

Summary:

On iOS, maintainVisibleContentPosition anchors on the first subview whose end is past the offset (#43203) and corrects by how far that view's origin moved. A view that straddles the top edge and is resized keeps its origin while everything after it moves, so nothing is corrected. After a prepend, that view is a VirtualizedList spacer being re-estimated, and the visible rows jump. This keeps #43203's anchor, but when it straddles the leading edge it also records the next subview, if that one starts inside the viewport, and measures it instead when the mount resized, recycled or removed the straddling anchor. Switching to "first fully visible" would also fix this, but it reverts #43203.

FlatList also needs #58916. With this fix alone, VirtualizedList unmounts the anchor and the row ends up 1828pt off.

Fixes #58913. Repro: https://github.com/mozzius/scrollview-mvcp-anchor-repro

Changelog:

[IOS] [FIXED] - maintainVisibleContentPosition corrects when the anchor straddling the top edge is resized, e.g. a VirtualizedList spacer after a prepend

Test Plan:

• Repro on an iPhone 17 Pro simulator (iOS 26.5, RN 0.87.1 with core built from source). Row 0's movement after a prepend with FlatList's default props:

  • stock: -962.3pt;
  • this fix alone: -1828.3pt;
  • this fix + Anchor VirtualizedList interior spacers to the cell before them #58916: +0.0pt, 3/3 runs with a list header and 3/3 without.

  With maxToRenderPerBatch={30}, this fix alone gives +0.0pt.

• Control, with rows inserted below a straddling row (Allow partially visible items to be picked as scroll anchors for maintainVisibleContentPosition #43203's case): +0.0pt both stock and with the fix. origin >= offset gives -450pt.

• Added three cases to RCTScrollViewComponentViewTests: a straddling anchor that is resized, one that is removed, and rows inserted below one. These only run inside Meta, so I also compiled the prepare/adjust methods verbatim into a macOS harness. The first two fail on main and pass with the fix; the third passes on both.

• clang++ -fsyntax-only against main's headers is clean for both files, and so is clang-format. This merges cleanly with Fix iOS maintainVisibleContentPosition skipping a clipped anchor #58912 in either order.

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