Own serialization options by the document's assembly

The static UntrustedData options are gone. An assembly declares its formatter
resolvers with [assembly: CultCacheFormatterResolver(typeof(R))];
CultDocumentMessagePackSerialization.OptionsFor(assembly) composes them ahead
of CultDocumentResolver and StandardResolver, once per assembly, and hands
assemblies that declare nothing the base Options. Generated codecs and the
reflective SerializeUntyped/DeserializeUntyped path both read
OptionsFor(typeof(X).Assembly). No registration call, no mutable options, no
load order.

CultMessagePackSecurity copies UntrustedData and supplies a collision-resistant
comparer over CultRecordRef<T>.Key.Value, so ref-keyed dictionaries
deserialize.

Tests: RefKeyedDictionaryRoundTrips failed on 825b4f7 ("No hash-resistant
equality comparer available"); DeclaredResolverEncodesValueTypeAsPositionalArray
and UnknownAssemblyGetsBaseOptions did not compile there (new API);
InteropNoteBytesUnchanged is a pin whose payload constant was captured on
825b4f7. BenchItem, the one unkeyed [CultDocument] in any consumer (in-memory
benchmark, no store), gains [Key]s in its existing slot order.

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

src/GameCult.Caching.MessagePack/CultCacheFormatterResolverAttribute.cs

@@ -0,0 +1,15 @@
+using System;
+
+namespace GameCult.Caching.MessagePack;
+
+// The resolver type exposes a public static Instance of IFormatterResolver or a public parameterless constructor.
+[AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true)]
+public sealed class CultCacheFormatterResolverAttribute : Attribute
+{
+    public CultCacheFormatterResolverAttribute(Type resolverType)
+    {
+        ResolverType = resolverType ?? throw new ArgumentNullException(nameof(resolverType));
+    }
+
+    public Type ResolverType { get; }
+}