Function generate_primary_xmp 

pub fn generate_primary_xmp(gainmap_length: usize) -> String

Generate XMP for the primary JPEG: Container:Directory only.

The primary image’s XMP contains hdrgm:Version and the Container:Directory listing every secondary with its semantic and byte length. The numeric gain-map metadata goes in the secondary’s XMP via generate_gainmap_xmp. This matches the layout produced by Google Camera / Android.