CultCache interop: C# and Rust write a union and nested bytes, the other reads them read-only

Extends the existing cross-runtime harness (cultcache-ts interop test, C# and Rust interop peers) with
write-shapes/read-shapes: a union field with two arms and bin members. Rust reads through the read-only
snapshot door; leftover staging files of both runtimes sit beside the store and must survive the read,
and the store bytes must not change.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/GameCult.Caching.MessagePack/GameCult.Caching.MessagePack.csproj

@@ -8,7 +8,7 @@
     <ProjectReference Include="..\GameCult.Caching\GameCult.Caching.csproj" />
     <ProjectReference Include="..\..\packages\cultmath\src\CultMath\CultMath.csproj" />
     <PackageReference Include="MessagePack" Version="3.1.7" />
-    <PackageReference Include="System.Text.Json" Version="8.0.5" />
+    <PackageReference Include="System.Text.Json" Version="9.0.10" />
     <PackageReference Include="MessagePackAnalyzer" Version="3.1.7" PrivateAssets="all" />
     <InternalsVisibleTo Include="GameCult.Caching.Tests" />
     <InternalsVisibleTo Include="GameCult.Caching.GeneratorConsumer" />

src/GameCult.Geometry/GameCult.Geometry.csproj

@@ -12,7 +12,7 @@
     <ProjectReference Include="..\GameCult.Caching\GameCult.Caching.csproj" />
     <ProjectReference Include="..\GameCult.Caching.MessagePack\GameCult.Caching.MessagePack.csproj" />
     <PackageReference Include="MessagePack" Version="3.1.7" />
-    <PackageReference Include="System.Text.Json" Version="8.0.5" />
+    <PackageReference Include="System.Text.Json" Version="9.0.10" />
   </ItemGroup>
 
   <Import Project="..\GameCult.Caching.MessagePack\buildTransitive\GameCult.Caching.MessagePack.targets" />

src/GameCult.Networking/GameCult.Networking.csproj

@@ -9,7 +9,7 @@
     <ProjectReference Include="..\GameCult.Caching.MessagePack\GameCult.Caching.MessagePack.csproj" />
     <PackageReference Include="MessagePack" Version="3.1.7" />
     <PackageReference Include="MessagePackAnalyzer" Version="3.1.7" PrivateAssets="all" />
-    <PackageReference Include="System.Text.Json" Version="8.0.5" />
+    <PackageReference Include="System.Text.Json" Version="9.0.10" />
     <PackageReference Include="R3" Version="1.3.0" />
     <PackageReference Include="LiteNetLib" Version="2.1.2" />
     <PackageReference Include="Isopoh.Cryptography.Argon2" Version="2.0.0" />