♻️ Adopt the three-overload matcher

strict/widened each take the exhaustive/fallback pattern shape in three
overloads ordered ExhaustiveLoose -> Fallback -> Handlers. TypeScript reads
the first overload for the object-literal popup (_?, a, b) and the last for
the missing-key error, so autocomplete and the error message are tuned
independently. The fallback is a pattern shape, not a factory, so the four
getPrimitiveUnionMatcher* factories collapse to two.

src/index.ts re-exports the two factories; the prototype scratch files that
explored the alternatives are removed.

src/index.ts

@@ -1,6 +1 @@
-export {
-    getPrimitiveUnionMatcher,
-    getPrimitiveUnionMatcherPartial,
-    getPrimitiveUnionMatcherPartialW,
-    getPrimitiveUnionMatcherW,
-} from "./primitive.ts";
+export { strict, widened } from "./primitive.ts";