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/HostLogonAutostartTests.cs

@@ -0,0 +1,19 @@
+using Explorer.Hosting;
+
+namespace Explorer.Hosting.Tests;
+
+public class HostLogonAutostartTests
+{
+    [Fact]
+    public void Create_args_are_per_user_logon_not_system_service()
+    {
+        var args = HostLogonAutostart.CreateTaskArgs(@"C:\Tools\Explorer.Host.exe");
+        Assert.Contains("/SC", args);
+        Assert.Contains("ONLOGON", args);
+        Assert.Contains("/RL", args);
+        Assert.Contains("LIMITED", args);
+        Assert.Contains(HostLogonAutostart.TaskName, args);
+        Assert.DoesNotContain("ONSTART", args);
+        Assert.DoesNotContain("/RU", args);
+    }
+}