Anchor VirtualizedList interior spacers to the cell before them

[mozzius]

Summary:

After a large prepend under maintainVisibleContentPosition, the spacer between the cells kept by initialNumToRender and the render window starts at averageCellLength * first rather than where the kept cells end. The cells after it then move each time a cell mounts or unmounts at the window's leading edge, and mVCP's correction moves that edge back, so the content height and offset swing by ~400pt about 15 times a second, indefinitely. This starts a spacer whose first cell is unmeasured where the rendered cell before it ends; only the start is anchored, because sizing it from the cell after it as well would include any gap/rowGap or CellRendererComponent margins and grow by twice that on every render.

Fixes #58870. Repro: https://github.com/mozzius/virtualizedlist-spacer-ring-repro

Changelog:

[GENERAL] [FIXED] - VirtualizedList: anchor an interior spacer's start to the rendered cell before it, so maintainVisibleContentPosition doesn't oscillate after a large prepend

Test Plan:

• New test in VirtualizedList-test.js, with and without a 10pt gap between cells: fails on main (spacer 1166.7 / 1176.7, expected 1000) and passes with the fix.

• yarn jest packages/virtualized-lists: 187 passed, all 69 snapshots unchanged. yarn flow-check, eslint and prettier are clean.

• The repro on an iPhone 17 Pro simulator (iOS 26.5, RN 0.87.1), 4 identical runs per row. Offset jumps the list made by itself after the prepend / while scrolling up in 50pt steps / in 5s at rest:

  	Stock	Fix
  No gap	3 / 6 / 75 (±449pt)	2 / 1 / 0
  gap: 10	3 / 7 / 75 (±398pt)	2 / 1 / 0

  Scrolling 3000pt up through the prepended rows: stock 121 jumps with 109 reversals, the fix 2 with none, with or without the gap. Sizing the spacer from both neighbours instead made 21 jumps of exactly +20pt (twice the gap).

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