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);

app/MindWork AI Studio/Tools/DroppedPaths.cs

@@ -0,0 +1,14 @@
+namespace AIStudio.Tools;
+
+/// <summary>
+/// Hands the dropped paths to the drop zone which was under the cursor.
+/// </summary>
+/// <remarks>
+/// The zone is named rather than addressed, because the message bus broadcasts. Only the zone whose
+/// own ID matches acts on this, and every other zone ignores it -- including the zones of circuits
+/// whose browser is long gone, because an ID belongs to one instance in one circuit. What the paths
+/// mean is the receiving zone's business: they may lead to files just as well as to folders.
+/// </remarks>
+/// <param name="ZoneId">The ID of the zone the paths were dropped on.</param>
+/// <param name="Paths">The dropped paths, in the order the runtime delivered them.</param>
+public readonly record struct DroppedPaths(string ZoneId, List<string> Paths);

app/MindWork AI Studio/Tools/Event.cs

@@ -222,6 +222,16 @@ public enum Event
     /// Unregisters a file drop area from file attachment handling.
     /// </summary>
     UNREGISTER_FILE_DROP_AREA,
+
+    /// <summary>
+    /// Names the drop zone under the cursor of a running drag so that exactly this one is highlighted.
+    /// </summary>
+    HIGHLIGHT_DROP_ZONE,
+
+    /// <summary>
+    /// Delivers dropped paths to the drop zone which was under the cursor.
+    /// </summary>
+    PATHS_DROPPED,