test: fix PhysicsResolveCapture/PhysicsDiagnostics static-leak isolation

xUnit's default parallel execution let diagnostic-harness tests (CellarUp,
DoorBug, DoorCollisionApparatus) mutate PhysicsResolveCapture.CapturePath
and PhysicsDiagnostics probe flags concurrently with victim tests
(MotionInterpreter, PositionManager, PlayerMovementController,
DispatcherToMovement, BSPStepUp), producing a flaky 14-26 failure range.

Fixes:
- Add PhysicsResolveCapture.ResetForTest() + PhysicsDiagnostics.ResetForTest()
  as documented test-only reset APIs (never called from production paths).
- Add IDisposable to CellarUpTrajectoryReplayTests with ctor/Dispose calling
  both ResetForTest() — prevents CapturePath from leaking between the Capture_*
  tests in the same class (the immediate root cause of Capture_SkipsNonPlayerCalls
  finding an unexpected file).
- Add xunit.runner.json (maxParallelThreads=1, parallelizeTestCollections=false)
  to AcDream.Core.Tests — eliminates parallelism-induced probe-flag leaks across
  all test classes without requiring [Collection] boilerplate on every offender.

After: two consecutive runs produce the identical 12-failure set.
Confirmed: LiveCompare_FirstCap_FixClosesCottageFloorCap passes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/PhysicsResolveCapture.cs

@@ -148,6 +148,24 @@ public static class PhysicsResolveCapture
     public static void ResetTickCounter() =>
         Interlocked.Exchange(ref _tickCounter, 0);
 
+    /// <summary>
+    /// Test-only reset: close any open writer, clear <see cref="CapturePath"/>,
+    /// and reset the tick counter to 0.  Call from test constructors and
+    /// <c>IDisposable.Dispose()</c> to prevent static state from leaking
+    /// across test-class boundaries.
+    ///
+    /// <para>
+    /// This method is intentionally <c>public</c> so test projects can call it
+    /// without reflection, but it must NEVER be called from production code paths.
+    /// </para>
+    /// </summary>
+    public static void ResetForTest()
+    {
+        Close();
+        CapturePath = null;
+        Interlocked.Exchange(ref _tickCounter, 0);
+    }
+
     private static void EnsureWriter_NoLock()
     {
         if (_writer is not null)