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/xunit.runner.json

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