test(agent): serialise V2.Core RecoveryPath / EnrollmentOrchestrator / CollectorBaseReentrancy

These three classes intermittently failed under the parallel xUnit pool
on a busy box (snapshot/journal IO + Task coordination + Interlocked-gate
SpinUntil all timing-sensitive against ThreadPool contention from
unrelated tests). Confirmed pattern, not real regressions: each class
ran green isolated via --filter.

Now decorated with [Collection("SerialThreading")] — same collection as
DeadlineScheduler, SignalIngress, EnrollmentOrchestratorMaxLifetime,
etc. Verified 5 consecutive full-suite runs green (855/855 each, ~28s).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Orchestration/EnrollmentOrchestratorTests.cs

@@ -18,6 +18,10 @@ using Xunit;
 
 namespace AutopilotMonitor.Agent.V2.Core.Tests.Orchestration
 {
+    // Persistence-IO + Task-coordination tests are timing-sensitive enough that the
+    // parallel xUnit pool starves the worker threads on a busy CI box. Serialised
+    // against the other threading-sensitive classes via SerialThreading.
+    [Collection("SerialThreading")]
     public sealed class EnrollmentOrchestratorTests
     {
         private static DateTime At => new DateTime(2026, 4, 20, 10, 0, 0, DateTimeKind.Utc);

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Orchestration/RecoveryPathTests.cs

@@ -20,6 +20,10 @@ using Xunit;
 
 namespace AutopilotMonitor.Agent.V2.Core.Tests.Orchestration
 {
+    // Snapshot/journal file IO + ThreadPool-driven persistence races make this class
+    // intermittently flaky under the parallel xUnit pool. Serialised against other
+    // threading-sensitive classes via the SerialThreading collection.
+    [Collection("SerialThreading")]
     public sealed class RecoveryPathTests
     {
         private static DateTime At => new DateTime(2026, 4, 20, 10, 0, 0, DateTimeKind.Utc);

src/Agent/AutopilotMonitor.Agent.V2.Core.Tests/Telemetry/CollectorBaseReentrancyTests.cs

@@ -28,7 +28,15 @@ namespace AutopilotMonitor.Agent.V2.Core.Tests.Telemetry
     /// gate behaviour so a future "let's just remove the guard, it never fires" refactor
     /// has a hard signal.
     /// </para>
+    /// <para>
+    /// Threading-sensitive: the test exercises the <see cref="Interlocked"/> gate via
+    /// real Tasks + SpinUntil. Under the parallel xUnit pool the ThreadPool gets
+    /// contended by unrelated tests and the SpinUntil times out before the gate
+    /// closes. Serialised against the other threading-sensitive classes via
+    /// SerialThreading.
+    /// </para>
     /// </summary>
+    [Collection("SerialThreading")]
     public sealed class CollectorBaseReentrancyTests
     {
         private static AgentLogger NewLogger(TempDirectory tmp)