[dfmc-modeling] Fix ^known-disjoint? ambiguity with singletons and unions

Without this, you could get the occasional warning:

````
    Warning: Method specificity of unorderable methods
      {<pair>:
        {<simple-method>: ??? (<&type>, <&union>)},
        {<simple-method>: ??? (<&singleton>, <&type>)}}
    applying
      {<sealed-generic-function>: ^known-disjoint?}
    to
      {<simple-object-vector>: {<&singleton>}, {<&union>}}
    was determined with arbitrary and capricious rules.
````