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>

Aetheria.Shared/Aetheria.Shared.csproj

@@ -3,7 +3,7 @@
        Compiles ServerShared and its dependencies from source with no Unity defines, so a successful build is the
        check that the simulation stays independent of the engine. Tools and test harnesses reference this project. -->
   <PropertyGroup>
-    <!-- Unity compiles ServerShared against .NET Standard 2.1; newer targets also impose C# 11 ref-safety rules the vendored MessagePack predates -->
+    <!-- Unity compiles ServerShared against .NET Standard 2.1 -->
     <TargetFramework>netstandard2.1</TargetFramework>
     <LangVersion>9.0</LangVersion>
     <Nullable>disable</Nullable>
@@ -18,7 +18,6 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="$(Assets)Scripts\ServerShared\**\*.cs" />
-    <Compile Include="$(Assets)Plugins\MessagePack\**\*.cs" Exclude="$(Assets)Plugins\MessagePack\Unity\**" />
     <Compile Include="$(Assets)Plugins\UniRx\Scripts\**\*.cs" Exclude="$(Assets)Plugins\UniRx\Scripts\UnityEngineBridge\**;$(Assets)Plugins\UniRx\Scripts\UnityWinRTBridge\**;$(Assets)Plugins\UniRx\Scripts\InternalUtil\MicroCoroutine.cs" />
     <!-- The reactive collections live in UniRx's Unity bridge but fence their Unity parts behind UniRxLibrary -->
     <Compile Include="$(Assets)Plugins\UniRx\Scripts\UnityEngineBridge\ReactiveProperty.cs;$(Assets)Plugins\UniRx\Scripts\UnityEngineBridge\ReactiveCollection.cs;$(Assets)Plugins\UniRx\Scripts\UnityEngineBridge\ReactiveDictionary.cs" />
@@ -27,7 +26,5 @@
   <ItemGroup>
     <ProjectReference Include="$(CultLibRoot)\packages\cultmath\src\CultMath\CultMath.csproj" />
     <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
-    <!-- Bundled by Unity; .NET Standard 2.1 lacks it -->
-    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="6.0.0" />
   </ItemGroup>
 </Project>