CultMesh: a raw write refuses a file whose header it cannot read

ExistingHeader read the whole snapshot and swallowed every failure into "no header": a
truncated v3 file got a v1 raw write and an unknown v9 file was overwritten, where
TypeScript, Python and Rust refuse. CultDocumentMessagePackSerialization.ReadStoreHeader
reads the prefix header, refuses a file that is not one complete MessagePack value or whose
format this runtime cannot read, and the raw write keeps a whole marked file marked.

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

src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs

@@ -1,4 +1,5 @@
 using System;
+using System.IO;
 using System.Buffers;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
@@ -240,6 +241,36 @@ public static class CultDocumentMessagePackSerialization
         }
     }
 
+    /// <summary>
+    /// The format header of a store file about to be replaced by a writer that does not read the store's records: null for
+    /// an empty file or a legacy envelope array, the header string otherwise. The header is read from the prefix, but a file
+    /// that is not one complete MessagePack value or whose header this runtime cannot read is refused, never taken for a
+    /// legacy file, so a writer cannot overwrite what it cannot see.
+    /// </summary>
+    public static string? ReadStoreHeader(byte[] bytes)
+    {
+        if (bytes.Length == 0)
+            return null;
+        try
+        {
+            var whole = new MessagePackReader(bytes);
+            whole.Skip();
+        }
+        catch (Exception ex) when (ex is EndOfStreamException or MessagePackSerializationException or InvalidOperationException)
+        {
+            throw new NotSupportedException("The store is not one complete MessagePack value; it is not rewritten.", ex);
+        }
+
+        var reader = new MessagePackReader(bytes);
+        if (reader.NextMessagePackType != MessagePackType.Array || reader.ReadArrayHeader() == 0 ||
+            reader.NextMessagePackType != MessagePackType.String)
+            return null;
+        var header = reader.ReadString();
+        if (header != null && header.StartsWith("cultcache.store.", StringComparison.Ordinal))
+            RequireSingleFileFormat(new CultPersistedStoreSnapshot { FormatVersion = header });
+        return header;
+    }
+
     private static void WritePersistedRecord(ref MessagePackWriter writer, CultPersistedRecord record)
     {
         writer.WriteArrayHeader(record.Variant == null ? PersistedRecordFieldCount : VariantRecordFieldCount);