Function operator_sum_preserves_particle_sector 

pub fn operator_sum_preserves_particle_sector<B>(
    basis: &B,
    terms: &[OperatorSpec],
) -> Result<bool>
where
    B: Basis,

Check additive-sector conservation for a complete operator sum.

Cartesian products are first decomposed into definite raising/lowering components. Coefficients are accumulated for identical spatial actions before sector closure is tested, so cancellations such as xx + yy are recognized. The Cartesian degree remains part of the canonical key because a basis may normalize x/y differently from explicit +/- symbols.