Reference CultMath instead of Unity.Mathematics

Aetheria.Shared.csproj drops the Library\PackageCache math sources and
references CultMath.csproj under the pinned CultLibRoot. The asmdefs drop
Unity.Mathematics; Tests overrides references, so it names CultMath.dll in
precompiledReferences. The manifest drops com.unity.mathematics (Burst and
Collections still resolve it) and adds org.gamecult.cultmath at
cultmath-unity-v0.2.1.

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

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

@@ -3,7 +3,6 @@
     "rootNamespace": "",
     "references": [
         "MessagePack",
-        "Unity.Mathematics",
         "UniRx",
         "Ink"
     ],