test: serialize AcDream.Core.Net.Tests so real-time transport waits are not starved

PausedSelector_SeededDroppedServerReady_RecoversOnIdleSweep failed twice in CI
on the Linux runner, taking 37 s and 42 s, while passing 5/5 in ~350 ms in
isolation on that same machine. The test drives a virtual clock but asserts on
real-time 2 s SpinUntil windows, so full-assembly parallelism on a 6-core
container starves it.

Passing -- xUnit.MaxParallelThreads=2 through dotnet test did not take effect.
A xunit.runner.json is read by xUnit directly and is the convention already
used by AcDream.Core.Tests and AcDream.Launcher.Tests.

tests/AcDream.Core.Net.Tests/AcDream.Core.Net.Tests.csproj

@@ -22,4 +22,15 @@
     <ProjectReference Include="..\..\src\AcDream.Core.Net\AcDream.Core.Net.csproj" />
   </ItemGroup>
 
+  <ItemGroup>
+    <!-- Several transport tests drive a virtual clock but assert on REAL-time
+         waits (2 s SpinUntil windows). Under default parallelism on a small CI
+         container PausedSelector_SeededDroppedServerReady_RecoversOnIdleSweep
+         took 37-42 s and missed its window, while passing 5/5 in ~350 ms in
+         isolation on the same machine. Serialize the assembly rather than
+         loosen a Campaign N transport assertion. Same settings
+         AcDream.Core.Tests and AcDream.Launcher.Tests already use. -->
+    <None Update="xunit.runner.json" CopyToOutputDirectory="PreserveNewest" />
+  </ItemGroup>
+
 </Project>

tests/AcDream.Core.Net.Tests/xunit.runner.json

@@ -0,0 +1,6 @@
+{
+  "$schema": "https://xunit.net/schema/current/xunit.runner.schema.json",
+  "parallelizeAssembly": false,
+  "parallelizeTestCollections": false,
+  "maxParallelThreads": 1
+}