Function parse_declared_sections 

pub fn parse_declared_sections<'a>(
    lines: &mut impl LineStream<'a>,
    header: &mut FrameHeader,
    atom_data: &mut [AtomDatum],
) -> Result<usize, ParseError>

Parses declared sections from a frame’s header metadata.

If header.sections is non-empty (v2 file with "sections" key in JSON), parses each declared section in order. Otherwise falls back to legacy blank-separator velocity detection.