docs(saves): note the failure mode if the type-table invariant guard fires

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Projects/Server/Serialization/GenericEntityPersistence.cs

@@ -293,6 +293,10 @@ public class GenericEntityPersistence<T> : GenericPersistence, IGenericEntityPer
 
     private ushort GetTypeIndex(T entity)
     {
+        // Every path into EntitiesBySerial registers the type first, so this cannot fire.
+        // If it ever does, the segment-level catch in WriteSnapshot logs it and moves on —
+        // the failed segment's records are dropped from the idx while binPosition rewinds,
+        // so treat any occurrence as a serious bug in an insertion path, not a bad entity.
         if (!_typeIndexes.TryGetValue(entity.GetType(), out var typeIndex))
         {
             throw new InvalidOperationException(