Pin Holds on every list shape, and drop C#'s redundant first-slot check

CultElementIds.Holds is the one fact a store marks its header by; a test now
drives it directly over a dictionary value, a list of lists, a list under an
unset element and an element, and over unset ids that hold nothing, so a
mutant that skips any of those branches fails.

C# ReadStoreHeader refused a first-slot string without the cultcache.store.
prefix and then ran RequireSingleFileFormat, which refuses every string that
is not v1, v2 or v3 anyway. The prefix check is gone; a non-header string is
refused as a format this runtime does not read.

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

src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs

@@ -270,9 +270,8 @@ public static class CultDocumentMessagePackSerialization
         var reader = new MessagePackReader(bytes);
         if (reader.ReadArrayHeader() == 0 || reader.NextMessagePackType != MessagePackType.String)
             return null;
+        // A first slot that is a string is a header, and one this runtime does not read is refused, whatever the string says.
         var header = reader.ReadString() ?? string.Empty;
-        if (!header.StartsWith("cultcache.store.", StringComparison.Ordinal))
-            throw new NotSupportedException($"The store's first slot is the string '{header}', not a store header; it is not rewritten.");
         RequireSingleFileFormat(new CultPersistedStoreSnapshot { FormatVersion = header });
         return header;
     }