Cut over the window to Explorer.Host.exe so only the host writes the index.

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

tests/Explorer.Storage.Tests/StorageTests.cs

@@ -451,4 +451,24 @@ public class IndexStoreLockTests
         Assert.Equal(IndexStoreLock.MutexNameFor(path), IndexStoreLock.MutexNameFor(path));
         Assert.StartsWith(@"Local\ExplorerWorkbench-Index-", IndexStoreLock.MutexNameFor(path));
     }
+
+    [Fact]
+    public async Task Read_only_store_opens_while_writer_holds_the_mutex()
+    {
+        var path = Path.Combine(Path.GetTempPath(), "ew-tests", Guid.NewGuid().ToString("N"), "index.db");
+        await using var writer = new SqliteIndexStore(path, NullLogger<SqliteIndexStore>.Instance);
+        await writer.OpenAsync();
+        await writer.AddSourceAsync(@"C:\data");
+
+        await using var reader = new SqliteIndexStore(path, NullLogger<SqliteIndexStore>.Instance, readOnly: true);
+        await reader.OpenAsync();
+        Assert.False(reader.CanWrite);
+        var sources = await reader.Sources.GetAllAsync();
+        Assert.Single(sources);
+        Assert.Equal(@"C:\data", sources[0].LastRootPath);
+
+        var write = await Assert.ThrowsAsync<InvalidOperationException>(
+            () => reader.RunWriteAsync(_ => Task.CompletedTask));
+        Assert.Contains("read-only", write.Message, StringComparison.OrdinalIgnoreCase);
+    }
 }