ci: cap xUnit parallelism on the constrained Linux runner

FakeAceTransportTests.PausedSelector_SeededDroppedServerReady_RecoversOnIdle
Sweep failed in CI after 37 s while passing 5/5 in ~350 ms in isolation on the
same machine: under full-assembly parallel load on a 6-core container its 2 s
real-time waits get starved. The test is timing-sensitive, not broken, so cap
the runner rather than edit Campaign N transport code.

.gitea/workflows/ci.yml

@@ -75,8 +75,15 @@ jobs:
             tests/AcDream.Launcher.Core.Tests \
             tests/AcDream.UI.Abstractions.Tests ; do
             echo "::group::$p"
+            # MaxParallelThreads is capped because this runner is a small
+            # container. Under full-assembly parallel load
+            # FakeAceTransportTests.PausedSelector_SeededDroppedServerReady_
+            # RecoversOnIdleSweep took 37 s and missed its 2 s real-time wait,
+            # while passing 5/5 in ~350 ms in isolation. Timing-sensitive, not
+            # broken: starve it less rather than edit Campaign N transport code.
             dotnet test "$p" -c Release --nologo \
-              --filter 'Lane!=InstalledDat&Lane!=PreparedPackage&Lane!=Live&Lane!=Manual&Lane!=Windows&Lane!=SystemFont&Purpose!=Diagnostic&Status!=KnownFailure'
+              --filter 'Lane!=InstalledDat&Lane!=PreparedPackage&Lane!=Live&Lane!=Manual&Lane!=Windows&Lane!=SystemFont&Purpose!=Diagnostic&Status!=KnownFailure' \
+              -- xUnit.MaxParallelThreads=2
             echo "::endgroup::"
           done