Deduplicate property overrides; Shipping carrier nullable

- Fix property enumeration to dedupe overridden/shadowed properties, ensuring only the most-derived is included (affects serialization and schema).
- Update TOON_ISSUES.md: close duplicate property issue, document root cause and resolution.
- Make CargoPartnerId nullable in Shipping and IShipping; clarify ToonDescription for carrier/truck/trailer fields.
- Refine ShippingDocument ToonDescription for clarity.
- Minor null-safety and style cleanup in GridShipping.razor.

AyCode.Core/Serializers/TypeMetadataBase.cs

@@ -363,14 +363,21 @@ public abstract class TypeMetadataBase
     [UnconditionalSuppressMessage("Trimming", "IL2080",
         Justification = "Same as IL2070 — currentType variable starts DAMs-annotated but loses annotation "
                       + "through the BaseType reassignment in the loop.")]
-    private static List<PropertyInfo> BuildPropertyList(
-        [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicProperties)] Type type,
-        bool requiresWrite)
+    private static List<PropertyInfo> BuildPropertyList([DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicProperties)] Type type, bool requiresWrite)
     {
         // Collect properties from inheritance hierarchy (derived -> base order)
         // Sorted alphabetically for deterministic property index ordering
         var allProperties = new List<PropertyInfo>();
 
+        // Dedup by name across inheritance levels. An overridden (or 'new'-shadowed) property is
+        // declared at EVERY level it appears in, so BindingFlags.DeclaredOnly returns it once per
+        // level — without this guard the derived AND base PropertyInfo would both be collected
+        // (duplicate wire field on the runtime path; duplicate @types entry in Toon). The derived->base
+        // walk means the most-derived declaration is seen first and wins. Mirrors the SGen path
+        // (AcBinarySourceGenerator.GetAllSerializablePropertySymbols) so the runtime and generated
+        // property orders stay bit-identical.
+        var seen = new HashSet<string>(StringComparer.Ordinal);
+
         for (var currentType = type; currentType != null && currentType != typeof(object); currentType = currentType.BaseType)
         {
             // Get properties declared at this level only
@@ -379,7 +386,8 @@ public abstract class TypeMetadataBase
                 .Where(p => p.CanRead &&
                             (!requiresWrite || p.CanWrite) &&
                             p.GetIndexParameters().Length == 0 &&
-                            !IsUnsupportedPropertyType(p.PropertyType))
+                            !IsUnsupportedPropertyType(p.PropertyType) &&
+                            seen.Add(p.Name))
                 .OrderBy(static p => p.Name, StringComparer.Ordinal);
 
             allProperties.AddRange(levelProperties);