Function avc_codec_string 

pub fn avc_codec_string(fourcc: &str, sps: &H264SpsInfo) -> String

H.264 codec string <fourcc>.PPCCLL per RFC 6381 §3.3 — hex bytes from the SPS: PP = profile_idc, CC = the packed constraint_set flags byte, LL = level_idc. fourcc is the sample-entry type: avc1 (parameter sets out-of-band in avcC) or avc3 (in-band). Example: High@L4.0 → avc1.640028.