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>

tests/Explorer.Hosting.Tests/WorkbenchPipeTests.cs

@@ -0,0 +1,76 @@
+using Explorer.Application;
+using Explorer.Contracts;
+using Explorer.Domain;
+using Explorer.Hosting.Ipc;
+using Microsoft.Extensions.Logging.Abstractions;
+
+namespace Explorer.Hosting.Tests;
+
+public class WorkbenchPipeTests
+{
+    [Fact]
+    public void Handle_forwards_indexing_and_transfer_calls()
+    {
+        var indexing = new FakeIndexing();
+        var transfers = new FakeTransfers();
+        var server = new WorkbenchPipeServer(
+            new WorkbenchHost(indexing, transfers),
+            new WorkbenchIpcOptions { PipeName = "ew-test" },
+            NullLogger<WorkbenchPipeServer>.Instance);
+
+        var ping = server.Handle(new IpcEnvelope { V = WorkbenchIpc.ProtocolVersion, Op = "Ping" });
+        Assert.True(ping.Ok);
+
+        var scan = server.Handle(new IpcEnvelope { V = WorkbenchIpc.ProtocolVersion, Op = "Indexing.EnqueueFullScan", N = 42 });
+        Assert.True(scan.Ok);
+        Assert.Equal(42, indexing.FullScanId);
+
+        var pause = server.Handle(new IpcEnvelope { V = WorkbenchIpc.ProtocolVersion, Op = "Transfers.PauseAll" });
+        Assert.True(pause.Ok);
+        Assert.True(transfers.PausedAll);
+
+        var paused = server.Handle(new IpcEnvelope { V = WorkbenchIpc.ProtocolVersion, Op = "Transfers.IsPaused" });
+        Assert.True(paused.Paused);
+    }
+
+    [Fact]
+    public void Handle_rejects_other_protocol_versions()
+    {
+        var server = new WorkbenchPipeServer(
+            new WorkbenchHost(new FakeIndexing(), new FakeTransfers()),
+            new WorkbenchIpcOptions(),
+            NullLogger<WorkbenchPipeServer>.Instance);
+        var reply = server.Handle(new IpcEnvelope { V = 99, Op = "Ping" });
+        Assert.False(reply.Ok);
+        Assert.Contains("99", reply.Error);
+    }
+
+    private sealed class FakeIndexing : IIndexingHost
+    {
+        public long FullScanId { get; private set; }
+        public event EventHandler<ScanProgress>? ProgressChanged = delegate { };
+        public void EnqueueFullScan(long sourceId) => FullScanId = sourceId;
+        public void EnqueueFolderScan(long sourceId, string pathRel) { }
+        public void EnqueueReconcile(long sourceId, string pathRel) { }
+        public void Cancel(long sourceId) { }
+    }
+
+    private sealed class FakeTransfers : ITransferHost
+    {
+        public bool PausedAll { get; private set; }
+        public event EventHandler? Changed = delegate { };
+        public event EventHandler<TransferJob>? JobFinished = delegate { };
+        public bool IsPaused => PausedAll;
+        public IReadOnlyList<TransferJob> Snapshot() => [];
+        public void PauseAll() => PausedAll = true;
+        public void ResumeAll() => PausedAll = false;
+        public void Pause(long jobId) { }
+        public void Resume(long jobId) { }
+        public void Retry(long jobId) { }
+        public void Cancel(long jobId) { }
+        public void Dismiss(long jobId) { }
+        public void ClearFinished() { }
+        public bool MoveUp(long jobId) => false;
+        public bool MoveDown(long jobId) => false;
+    }
+}