Function php_instance_linear 

pub fn php_instance_linear(m: usize) -> SymmetricInstance

Pigeonhole at scale m, linear encoding: each pigeon row as the degree-1 generator 1 + Σ_h x_{p,h} (never dropped by a degree budget — the fixed-degree questions stay real at every m), plus the hole at-most-one pairs. Same symmetry and atoms as the clause encoding; pigeon-row generators anchored by their pigeon.