Function eval_unfused_block 

pub fn eval_unfused_block(
    fb: &FusedBlock,
    acts: &[i64],
    m: usize,
) -> (Vec<i64>, usize)

The UNFUSED reference: the same computation with each stage materialized into its own buffer (a DRAM round-trip per stage). Equal to eval_fused_block — the composition proof — but it writes 2·m·n intermediate elements the fused pass never does. Returns (result, intermediate_writes).