Ship S2 fixes: RequireDesigns refuses nulls explicitly, and a failing Compose falls back to the shipped catalog

- RequireDesigns names the null (entity, item, zone record, lot, lot ledger) instead of catching NullReferenceException.
- ResolveCatalog reports a Compose-level failure as an exclusion covering all mods and boots the shipped catalog.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Assets/Scripts/ServerShared/EntitySerializer.cs

@@ -128,6 +128,7 @@ public static class EntitySerializer
     // docking bay units, and their stored contents. The provenance ledger's GC root walk.
     public static IEnumerable<ItemInstance> Items(EntityPack pack)
     {
+        if (pack == null) throw new InvalidOperationException("This run's save is malformed: an entity is null. Start a new game.");
         yield return pack.Hull;
         foreach (var (_, item) in pack.Equipment) yield return item;
         foreach (var (_, item) in pack.CargoBays) yield return item;