♻️ Share the Matchable universe between both matchers

src/matcher-shared.ts

@@ -8,6 +8,12 @@ import type { ValueOf } from "type-fest";
 // A handler: one universe member in, one return value out.
 export type UnaryFn<T, R> = (shape: T) => R;
 
+// The primitive universe a matcher can discriminate. The primitive-union matcher
+// uses it directly; the tagged-union matcher uses it as the set of allowed
+// discriminant (`Tag`) values. `boolean` is admitted as the pair `true | false`;
+// see README § Caveats for the unsupported members.
+export type Matchable = string | number | boolean | null | undefined;
+
 // `boolean`, `null` and `undefined` cannot be property keys, so a mapped type
 // over a universe that includes one keys each such member by its
 // stringification. `PatternParam` inverts that projection, so a handler callback