cleanup: archive legacy-orders and placeholders, remove tombstone and backup files

tests/_archive/placeholders/Integration_PlaceholderTests.cs

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+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace NT8.Integration.Tests
+{
+    [TestClass]
+    public class PlaceholderIntegrationTests
+    {
+        [TestMethod]
+        public void Placeholder_ShouldPass()
+        {
+            // Placeholder test to make project compile
+            Assert.IsTrue(true);
+        }
+    }
+}

tests/_archive/placeholders/Performance_PlaceholderTests.cs

@@ -0,0 +1,15 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace NT8.Performance.Tests
+{
+    [TestClass]
+    public class PlaceholderPerformanceTests
+    {
+        [TestMethod]
+        public void Placeholder_ShouldPass()
+        {
+            // Placeholder test to make project compile
+            Assert.IsTrue(true);
+        }
+    }
+}