Fix WrapPanel drop index calculation near row boundaries

[pyking-wang]

Summary

• calculate WrapPanel targets from the drag proxy in the ItemsPanelRoot coordinate space
• group WrapPanel slots into flow lines so row-boundary and terminal positions are resolved correctly
• keep the translucent preview aligned with the final same-collection drop index during AutoScroll
• limit the change to the Sortable.Avalonia core library

Validation

• dotnet build Sortable.Avalonia/Sortable.Avalonia.csproj --no-restore -c Release
• verified same-collection WrapPanel moves 6 -> 4, 12 -> 6 after scrolling, and 3 -> 10
• verified preview index equals the final collection index and placeholder coordinates match the target slot

Fixes #21