Safety and Efficacy of AAV9/AP4B1 For Patients with AP4B1-related Hereditary Spastic Paraplegia Type 47 (SPG47): A Phase 1/2 Single-Center, Open-Label Study of Stereotactic Intra-cisterna Magna Administration. The goal of this clinical trial is to evaluate whether a gene therapy can safely treat children with SPG47, a rare genetic condition that causes progressive spasticity and developmental delays. The main questions it aims to answer are: * Is the gene therapy safe and well tolerated? * Does the gene therapy improve motor function and developmental outcomes? Participants will: * Undergo screening assessments to confirm eligibility * Receive a single dose of the gene therapy vector * Attend follow-up visits for safety monitoring and developmental assessments over the course of five years