Function coerce_arg_known 

pub(super) fn coerce_arg_known(
    arg: &Expr,
    expected: WgslType,
    got: WgslType,
    ctx: &mut WalkCtx<'_>,
)

Variant that takes the already-known operand type. Use this when an outer pass (binop_vec) has already type-walked the expression and may have emitted truncation edits that change the effective type — the stateless infer_type doesn’t see those edits and would over-coerce.