Allow unions within a domain like boolean to be used as discriminants

[ssalbdivad]

Currently the following is not discriminated during traversal:

const t = type(["string"])
  .pipe((s) => s.length)
  .or(["boolean"]);

This is because boolean is being treated as a union and we don't identify that we could discriminate using domain.

It should be discriminated similarly to the existing test case "discriminable tuple union" where number is substituted for boolean:

const $ = scope({
  a: () => $.type(["string"]).pipe((s) => [...s, "!"]),
  b: ["number"],
  c: () => $.type("a|b"),
});
const types = $.export();

attest<[number] | ((In: [string]) => Out<string[]>)>(types.c.t);
const expectedSerializedMorphs =
  types.a.raw.assertHasKind("morph").serializedMorphs;

attest(types.c.raw.assertHasKind("union").discriminantJson).snap({
  kind: "domain",
  path: ["0"],
  cases: {
    '"number"': {
      sequence: { prefix: ["number"] },
      proto: "Array",
      exactLength: 1,
    },
    '"string"': {
      in: {
        sequence: { prefix: ["string"] },
        proto: "Array",
        exactLength: 1,
      },
      morphs: expectedSerializedMorphs,
    },
  },
});