fix(scroll): maintain scroll position when prepending large items

[naqvitalha]

Summary

• When large items are prepended to a FlashList, the scroll correction mechanism needs multiple render passes to converge as unmeasured item heights settle. After the first correction pass, the anchor item could move outside the engaged indices window due to height estimation errors. The fallback full-data search was gated behind hasDataChanged, which was already false by the second pass — so the anchor was lost and scroll stayed wrong.
• Adds an isRefiningCorrection flag that keeps subsequent correction passes able to locate the anchor item and update the internal scroll offset until convergence. The flag piggybacks on the existing ignoreScrollEvents timeout for cleanup.

Test plan

• Open the "Prepend Maintain Scroll" example in the fixture app (or use the Expo Snack from the issue)
• Verify that after prepending large items, the original (green) item remains visible
• Verify the Chat example's "Add at Top" still maintains scroll position
• yarn test — all 183 unit tests pass

Fixes #2136