Function find_solid_block_below_air 

pub(crate) fn find_solid_block_below_air<N: DimensionNoises>(
    cache: &mut N::ColumnCache,
    noises: &N,
    aquifer: &mut Aquifer<N>,
    block_x: i32,
    block_z: i32,
    start_y: i32,
    min_solid_y: i32,
) -> Option<i32>

Finds the highest solid block below air in a single base-noise column.

Used by structure placement probes such as nether fossils. This preserves the same base terrain classification as repeated column_state calls, but shares the eight cell-corner density evaluations across adjacent Y positions.