Add the drop zone arbiter, still without consumers

app/MindWork AI Studio/Tools/DropZoneHighlight.cs

@@ -0,0 +1,12 @@
+namespace AIStudio.Tools;
+
+/// <summary>
+/// Names the drop zone under the cursor of a running drag.
+/// </summary>
+/// <remarks>
+/// Every zone receives this and compares the ID with its own: at most one zone is highlighted at a
+/// time, and all others have to give their highlight up. A null ID means that the cursor is over no
+/// zone at all, or that the drag has ended.
+/// </remarks>
+/// <param name="ZoneId">The ID of the zone under the cursor, or null when there is none.</param>
+public readonly record struct DropZoneHighlight(string? ZoneId);