Function fill_cell_corner_densities_4x 

pub fn fill_cell_corner_densities_4x(
    noises: &EndNoises,
    cache: &EndColumnCache,
    x: i32,
    ys: f64x4,
    z: i32,
    blended_noise_value_v: f64x4,
    out: &mut [f64],
)

SIMD form of fill_cell_corner_densities that batches 4 cell-corner Y values at fixed (x, z).

out layout: lane-major SoA. Lane i’s INTERPOLATED_COUNT channels live at out[i * INTERPOLATED_COUNT..(i + 1) * INTERPOLATED_COUNT]. out must have length 4 * INTERPOLATED_COUNT.

Per-lane semantics are bit-identical to four scalar fill_cell_corner_densities calls at the same Y values.