Stop MessagePack's source generator flowing to consumers

A direct MessagePackAnalyzer reference with PrivateAssets="all" in
Caching.MessagePack and Networking keeps MessagePack 3's generator out of
consumer builds, where it emitted a reference to a nonexistent
GeneratedMessagePackResolver formatter for Dictionary<CultRecordRef<T>, float>
(CS0426). CultLib composes StandardResolver itself and never registers a
generated resolver.

tests/GameCult.Caching.ConsumerBuild is RefKeyedDictionaryCompilesInAConsumer:
its build is the test. On 825b4f7 it failed with CS0426; now it builds and
obj has no MessagePack.SourceGenerator output.

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

tests/GameCult.Caching.ConsumerBuild/Program.cs

@@ -0,0 +1,14 @@
+using System.Collections.Generic;
+using GameCult.Caching;
+using MessagePack;
+
+// RefKeyedDictionaryCompilesInAConsumer: this project building is the test.
+System.Console.WriteLine(new ConsumerFaction().Standing.Count);
+
+[CultDocument("tests.consumer_faction", "tests.consumer_faction.v1")]
+[MessagePackObject]
+public sealed class ConsumerFaction
+{
+    [Key(0)] [CultName] public string Name { get; set; } = string.Empty;
+    [Key(1)] public Dictionary<CultRecordRef<ConsumerFaction>, float> Standing { get; set; } = new();
+}