[#12] NativeStructSizesTests: document why sizes are duplicated

Reviewer suggested sharing the storage-size literals between
Iox2NativeMethods.cs and this test file. Doing so would erase the
tripwire: Marshal.SizeOf<T> reads the StructLayout(Size = ...) attribute,
so if both sides derive from the same const, the assert collapses to
const-equals-const and trivially passes. The double-entry bookkeeping is
the point — a maintainer who updates one side but forgets the other
gets a failing test. Adding an in-line note so the design intent is
visible at the literals, not buried in the class docstring.

Author: patdhlk

tests/Ffi/NativeStructSizesTests.cs

@@ -27,6 +27,12 @@ namespace Iceoryx2.Tests.Ffi;
 /// </summary>
 public class NativeStructSizesTests
 {
+    // INTENTIONAL DUPLICATION of the Size attribute in Iox2NativeMethods.cs.
+    // Sharing a single const between the StructLayout Size and the test would
+    // make the assert trivially pass (both sides derive from the same value)
+    // and erase the tripwire. The point of the duplication is double-entry
+    // bookkeeping against the C header: if a maintainer updates one side but
+    // forgets the other, this test fails.
     private const int ServiceBuilderStorageSize = 9104;   // iceoryx2.h iox2_service_builder_storage_t
     private const int NodeBuilderStorageSize = 18696;     // iceoryx2.h iox2_node_builder_storage_t