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.Application.Tests/RemovableAutoIndexPlannerTests.cs

@@ -0,0 +1,73 @@
+using Explorer.Application;
+using Explorer.Domain;
+using Explorer.Windows;
+
+namespace Explorer.Application.Tests;
+
+public class RemovableAutoIndexPlannerTests
+{
+    [Fact]
+    public void Does_nothing_when_disabled()
+    {
+        var sources = new[] { Removable(1, indexed: false) };
+        Assert.Empty(RemovableAutoIndexPlanner.SourceIdsToScan(sources, autoIndexRemovable: false));
+    }
+
+    [Fact]
+    public void Queues_online_unindexed_removable_only()
+    {
+        var sources = new[]
+        {
+            Removable(1, indexed: false),
+            Removable(2, indexed: true),
+            Removable(3, indexed: false, status: SourceStatus.Offline),
+            Cloud(4),
+            Local(5)
+        };
+        Assert.Equal(new[] { 1L }, RemovableAutoIndexPlanner.SourceIdsToScan(sources, autoIndexRemovable: true));
+    }
+
+    private static Source Removable(long id, bool indexed, SourceStatus status = SourceStatus.Online)
+        => new()
+        {
+            Id = id,
+            StableKey = id.ToString(),
+            Kind = SourceKind.Removable,
+            DisplayName = "USB",
+            LastRootPath = @"E:\",
+            Status = status,
+            LastIndexedUtc = indexed ? DateTimeOffset.UtcNow : null
+        };
+
+    private static Source Cloud(long id)
+        => new()
+        {
+            Id = id,
+            StableKey = "cloud",
+            Kind = SourceKind.Cloud,
+            DisplayName = "Cloud",
+            LastRootPath = @"C:\Users\me\OneDrive",
+            Status = SourceStatus.Online
+        };
+
+    private static Source Local(long id)
+        => new()
+        {
+            Id = id,
+            StableKey = "local",
+            Kind = SourceKind.NtfsLocal,
+            DisplayName = "Local",
+            LastRootPath = @"C:\",
+            Status = SourceStatus.Online
+        };
+}
+
+public class ElevatedScanServiceTests
+{
+    [Fact]
+    public void Never_requests_elevation()
+    {
+        IElevatedScanService service = new WindowsElevatedScanService();
+        Assert.False(service.CanRequestElevation);
+    }
+}