Function grid_is_combinational 

pub fn grid_is_combinational(g: &Grid) -> bool

True iff every placed tile is Arith/Select (or pure routing) — i.e. the grid is fully COMBINATIONAL and SAT-lowerable. A Func (division/float) tile has no bitblast lowering, so such a grid is proven by eval ≡ VM + iverilog, not by certify_reconfig/certify_config_table; those refuse a non-combinational grid rather than panic opaquely inside the SAT lowering.