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IsRunning flips before the restore goroutine parks in StopStack; under parallel load the ==1 assertion raced it and read 0. Wait for the first stop to land, then assert — the red-proof property (no-fast-path mutant → count 2) is preserved. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01PSK5g6qYLknKj8u3QAFEr6