Function build_flush_source 

fn build_flush_source(
    schema: &SchemaRef,
    ranges: Vec<MemtableRange>,
    metadata: &RegionMetadataRef,
    options: &RegionOptions,
    field_column_start: usize,
) -> Result<(FlatSource, SequenceNumber)>

Builds one sequence-contiguous source, decoding encoded ranges if their intervals overlap other ranges. JSON2 layouts are reconciled for this group.