Indexed large data sources in steps so the others get their turn (#1052)

app/MindWork AI Studio/Tools/Services/DataSourceQueueLane.cs

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+namespace AIStudio.Tools.Services;
+
+/// <summary>
+/// The lanes of the queue of indexing runs, cf. DataSourceRunQueue.
+/// </summary>
+/// <remarks>
+/// Every run in the front lane comes out before any run in the back lane. DataSourceEmbeddingService
+/// picks the lane when it enqueues a run: a data source which carries on a pass of its indexing waits
+/// in the back lane, cf. IndexingPass. A mailbox therefore waits for one step at most, rather than for
+/// hours behind a large folder, and large data sources take turns with each other.
+/// </remarks>
+internal enum DataSourceQueueLane
+{
+    /// <summary>
+    /// For a data source which begins a pass, e.g. a mailbox at its interval.
+    /// </summary>
+    FRONT,
+
+    /// <summary>
+    /// For a data source which carries on a pass, after it did its share of the work.
+    /// </summary>
+    BACK,
+}