The definition of proper time as the time measured by a clock following a worldline and the parameterization of motion relative to it is a foundational textbook definition in special and general relativity, requiring no external citation.
The claim states a fundamental definition of proper time and its relation to worldlines in special relativity. This is a basic definition taught in introductory physics, making it common knowledge by definition and everyday pedagogical observation. Therefore, Step 0 classifies it as COMMON_KNOWLEDGE with no papers attached.