Function payload_section_start 

pub fn payload_section_start(path: &str) -> Result<u64, CnResult>

Byte offset within a blob file at which its payload section begins. Reads only the header; the disk-backed streaming source uses it to turn a PayloadLocator offset into an absolute file offset. Used only by the Metal-driven disk-backed streaming source for now (Vulkan/DirectX streaming catch-up is a follow-up).