Extract a per-user background host so indexing can later run outside the window.

Keep the GUI as the index writer for now; mutex, named pipe, and opt-in logon autostart prepare Explorer.Host.exe without two SQLite writers.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/Explorer.Contracts/IWorkbenchHost.cs

@@ -0,0 +1,36 @@
+using Explorer.Domain;
+
+namespace Explorer.Contracts;
+
+public interface IWorkbenchHost
+{
+    IIndexingHost Indexing { get; }
+    ITransferHost Transfers { get; }
+}
+
+public interface IIndexingHost
+{
+    event EventHandler<ScanProgress>? ProgressChanged;
+    void EnqueueFullScan(long sourceId);
+    void EnqueueFolderScan(long sourceId, string pathRel);
+    void EnqueueReconcile(long sourceId, string pathRel);
+    void Cancel(long sourceId);
+}
+
+public interface ITransferHost
+{
+    event EventHandler? Changed;
+    event EventHandler<TransferJob>? JobFinished;
+    bool IsPaused { get; }
+    IReadOnlyList<TransferJob> Snapshot();
+    void PauseAll();
+    void ResumeAll();
+    void Pause(long jobId);
+    void Resume(long jobId);
+    void Retry(long jobId);
+    void Cancel(long jobId);
+    void Dismiss(long jobId);
+    void ClearFinished();
+    bool MoveUp(long jobId);
+    bool MoveDown(long jobId);
+}