Act as a certified strength and conditioning coach with 10+ years of experience in calisthenics and bodyweight training. Provide a detailed, step-by-step guide on how to train for a one-arm pull-up, including [BEGINNER], [INTERMEDIATE], and [ADVANCED] progressions. Explain the importance of [GRIP STRENGTH], [CORE STABILITY], and [CONTROLLED ECCENTRICS] in mastering this skill. Include recommended exercises, rest periods, and frequency of training. Tailor the advice for someone with [CURRENT FITNESS LEVEL] and [GOAL TIMEFRAME].