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>

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

@@ -18,6 +18,15 @@
     <Using Include="Xunit" />
   </ItemGroup>
 
+  <ItemGroup>
+    <!-- T0: force serial test execution to prevent PhysicsResolveCapture /
+         PhysicsDiagnostics static state from leaking between test classes.
+         These statics are mutated by diagnostic-harness tests; xUnit's
+         default parallel execution races them against victim tests like
+         MotionInterpreterTests and PlayerMovementControllerTests. -->
+    <None Update="xunit.runner.json" CopyToOutputDirectory="PreserveNewest" />
+  </ItemGroup>
+
   <ItemGroup>
     <ProjectReference Include="..\..\src\AcDream.Core\AcDream.Core.csproj" />
     <ProjectReference Include="..\..\src\AcDream.App\AcDream.App.csproj" />

tests/AcDream.Core.Tests/Physics/CellarUpTrajectoryReplayTests.cs

@@ -80,8 +80,25 @@ namespace AcDream.Core.Tests.Physics;
 /// trajectory shape.
 /// </para>
 /// </summary>
-public class CellarUpTrajectoryReplayTests
+public class CellarUpTrajectoryReplayTests : IDisposable
 {
+    // ── T0: constructor + Dispose reset PhysicsResolveCapture static ────────
+    // xUnit creates a fresh instance per test, so the constructor runs before
+    // each test and Dispose runs after. Resetting here guarantees that no
+    // prior test in this class can leave CapturePath set (which would cause
+    // Capture_SkipsNonPlayerCalls to find an unexpected file).
+    public CellarUpTrajectoryReplayTests()
+    {
+        PhysicsResolveCapture.ResetForTest();
+        PhysicsDiagnostics.ResetForTest();
+    }
+
+    public void Dispose()
+    {
+        PhysicsResolveCapture.ResetForTest();
+        PhysicsDiagnostics.ResetForTest();
+    }
+
     // ── Cellar / cottage geometry constants ────────────────────────
     private const uint CellarId         = 0xA9B40147u;
     private const uint CottageNeighborA = 0xA9B40143u;

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

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