Add BitfieldChangeDetector stub for test project compilation

The source generator now emits a change detection factory that
references BitfieldChangeDetector<T>, which lives in the consuming
project. The test project needs a minimal stub so the generated
code compiles.

Author: mattd-bp

Modspec.Test/BitfieldChangeDetectorStub.cs

@@ -0,0 +1,13 @@
+/*
+ * Stub so the generated change detection factory compiles in the test project,
+ * which does not reference the real BitfieldChangeDetector implementation.
+ */
+namespace Modspec.Model;
+
+public class BitfieldChangeDetector<TClient>
+{
+    public BitfieldChangeDetector<TClient> Track<T>(System.Func<TClient, T> getter, System.Func<T, Level> getLevel) where T : struct, System.Enum
+    {
+        return this;
+    }
+}