Function parse_annexb 

pub fn parse_annexb(buf: &[u8]) -> HevcHdrSei

Scan an Annex-B byte buffer for HEVC SEI NAL units (nal_unit_type 39 prefix, 40 suffix) and extract HDR static metadata payloads. Returns a potentially-empty HevcHdrSei; callers should fold it into ColorMetadata only when non-empty.