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/CoreRegistrationTests.cs

@@ -0,0 +1,48 @@
+using Explorer.Contracts;
+using Explorer.Domain;
+using Explorer.Domain.Abstractions;
+using Explorer.Hosting;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Explorer.Hosting.Tests;
+
+public class CoreRegistrationTests
+{
+    [Fact]
+    public async Task AddExplorerCore_registers_workbench_without_clipboard()
+    {
+        var dir = Path.Combine(Path.GetTempPath(), "ew-hosting", Guid.NewGuid().ToString("N"));
+        Directory.CreateDirectory(dir);
+        try
+        {
+            var services = new ServiceCollection();
+            services.AddLogging();
+            services.AddSingleton<IAppEnvironment>(new TempEnv(dir));
+            services.AddExplorerCore();
+            await using var sp = services.BuildServiceProvider();
+            Assert.NotNull(sp.GetService<IWorkbenchHost>());
+            Assert.NotNull(sp.GetService<IIndexingHost>());
+            Assert.NotNull(sp.GetService<ITransferHost>());
+            Assert.Null(sp.GetService<IOsClipboard>());
+        }
+        finally
+        {
+            try { Directory.Delete(dir, true); } catch { /* ignore */ }
+        }
+    }
+
+    private sealed class TempEnv : IAppEnvironment
+    {
+        public TempEnv(string dir)
+        {
+            DataDirectory = dir;
+            DatabasePath = Path.Combine(dir, "index.db");
+            LogDirectory = Path.Combine(dir, "logs");
+            Directory.CreateDirectory(LogDirectory);
+        }
+
+        public string DataDirectory { get; }
+        public string DatabasePath { get; }
+        public string LogDirectory { get; }
+    }
+}