Function bool_shape_select_literals 

fn bool_shape_select_literals(
    e: &Expr,
    ctx: &mut WalkCtx<'_>,
) -> (&'static str, &'static str)

Pick the (zero, one) literals for a select(0.0/vecN, …) wrap based on the shape of the bool-producing expression. A scalar comparison (e.g. rs.z > 0) keeps the scalar 0.0/1.0; a vector comparison (e.g. noise >= 0.1) needs vecN<f32>(0.0)/vecN<f32>(1.0) so the resulting select(...) typechecks under WGSL’s strict-shape rules.