fix: resolve @observable double-free crash in tests

Release old OnboardingViewModel before creating a new instance in
testReInstantiatingAfterCompletionStartsAtWelcome to avoid malloc
double-free from @observable's internal tracking.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: nathanialhenniges

apps/native/WolfWaveTests/OnboardingViewModelEdgeCaseTests.swift

@@ -92,6 +92,9 @@ final class OnboardingViewModelEdgeCaseTests: XCTestCase {
         viewModel.completeOnboarding()
         XCTAssertTrue(viewModel.showCompletion)
 
+        // Release old instance before creating new one to avoid @Observable double-free
+        self.viewModel = nil
+
         // Create a new instance
         let newViewModel = OnboardingViewModel()
         XCTAssertEqual(newViewModel.currentStep, .welcome)