Delete the in-tree cache, JsonKnownTypes and vendored MessagePack

Cut 8b, deletes first. The old authorities are removed, not demoted:
- ServerShared/CultCache: CultCache.cs, DatabaseEntry.cs (root [Union],
  DatabaseLink, INamedEntry), RegisterResolver.cs, TypeFormatterResolver.cs,
  JsonConverters.cs, JsonKnownTypes/**.
- Assets/Plugins/MessagePack/** and System.Runtime.CompilerServices.Unsafe.dll
  (org.gamecult.cultlib ships both).
- GalaxyMapLayerData, PlayerData, GlobalSettingsAttribute.
- The inspector attributes CultLib already carries or nothing reads.

Kept and moved: MathFormatters.cs and MathResolver.cs to
ServerShared/Serialization; GetAllChildClasses, SplitCamelCase,
FormatTypeName, MaxBy and MinBy to Extensions.cs; Inspectable,
PreferredInspectorAttribute and the five Aetheria drawer attributes to
ServerShared/Attributes.cs. Aetheria.Shared.csproj drops the vendored
MessagePack include and the Unsafe package; the asmdef drops MessagePack.

Does not build: the data model moves onto CultLib in the next commit.

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

Assets/Scripts/ServerShared/Aetheria.Shared.Unity.asmdef

@@ -2,7 +2,6 @@
     "name": "Aetheria.Shared.Unity",
     "rootNamespace": "",
     "references": [
-        "MessagePack",
         "UniRx",
         "Ink"
     ],