Function pp_wave_diagonal 

pub fn pp_wave_diagonal(
    stages: usize,
    waves: usize,
    diagonal: usize,
) -> Vec<(usize, usize)>

Cells on one pipeline anti-diagonal, returned as (wave, stage). Cells in a diagonal never share a wave (request/cache state) or a stage (Engine scratch/stream), so they may be driven by scoped host threads without reintroducing the shared-Engine race that quarantined the old deferred PP walker.