C#: compatible schema ids publish a schema on open, Mesh takes only the older layout, the refusal names its file

A record whose schema id the catalog lists only as a compatible id now opens (the cache resolved a persisted schema by entry id
alone). CultMesh's fallback to the older catalog-entry reader fires only for a store in that layout, decided by the entry's slot 5,
not for any decode failure, so a current-layout store with a malformed slot is refused as CultCache refuses it.
CultStoreUnreadableException.Path names the refused file. New vectors: compatible-id-only-v3 (accepted by all four) and three
current-layout catalogs with one malformed slot. The contract states the writer's invariant.

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

src/GameCult.Caching.MessagePack/DirectoryMessagePackBackingStore.cs

@@ -451,7 +451,16 @@ public sealed class DirectoryMessagePackBackingStore : CacheBackingStore
             };
         }
 
-        var snapshot = CultDocumentMessagePackSerialization.DeserializeSnapshot(manifestBytes);
+        CultPersistedStoreSnapshot snapshot;
+        try
+        {
+            snapshot = CultDocumentMessagePackSerialization.DeserializeSnapshot(manifestBytes);
+        }
+        catch (CultStoreUnreadableException ex) when (ex.Path == null)
+        {
+            throw new CultStoreUnreadableException(ex.Message, _manifestFile.FullName, ex.InnerException);
+        }
+
         if (!string.Equals(snapshot.FormatVersion, IndexedFormatVersion, StringComparison.Ordinal) &&
             !string.Equals(snapshot.FormatVersion, IndexedFormatVersionWithIds, StringComparison.Ordinal))
         {