ci: run Core.Net single-threaded on Linux, as its own step

Push 2 went red on a SIXTH member of the load-sensitive family
(PrunedId_RejectAtFreshSequence_ReclaimKeepsTheStreamAligned), with the
assembly taking 59 s on Linux against ~7 s on Windows. Laning members one at a
time was the pattern that already failed to converge, so this addresses the
assembly instead.

The split is measured, not defensive: on this 6-core container Core.Net FAILS
in 40 s with default parallelism and PASSES in 10 s single-threaded. Its
sessions do real socket work on background threads, so contention both breaks
and slows them. Windows keeps default parallelism — 18 cores, ~7 s, and it
REGRESSED when the same assembly was serialized there.

An earlier attempt passed the flag through a bash array inside the shared loop
and never reached dotnet. This gives the project its own explicit invocation so
the flag cannot be swallowed.

.gitea/workflows/ci.yml

@@ -65,10 +65,23 @@ jobs:
         run: |
           set -e
           dotnet --version
+          # Core.Net runs SINGLE-THREADED here, on its own, and the split is
+          # measured rather than defensive: on this 6-core container the
+          # assembly FAILS in 40 s with default parallelism and PASSES in 10 s
+          # with one thread. Its sessions do real socket work on background
+          # threads, so contention both breaks and slows them. Windows has 18
+          # cores, passes in ~7 s parallel, and REGRESSED when serialized, so
+          # this stays scoped to Linux.
+          echo '::group::AcDream.Core.Net.Tests (single-threaded)'
+          dotnet test tests/AcDream.Core.Net.Tests/AcDream.Core.Net.Tests.csproj \
+            -c Release --nologo \
+            --filter 'Lane!=InstalledDat&Lane!=PreparedPackage&Lane!=Live&Lane!=Manual&Lane!=Timing&Lane!=Windows&Lane!=SystemFont&Purpose!=Diagnostic&Status!=KnownFailure' \
+            -- xUnit.MaxParallelThreads=1
+          echo '::endgroup::'
+
           for p in \
             tests/AcDream.Platform.Tests \
             tests/AcDream.Core.Tests \
-            tests/AcDream.Core.Net.Tests \
             tests/AcDream.Content.Tests \
             tests/AcDream.Runtime.Tests \
             tests/AcDream.Headless.Tests \