🐛 Infer strict return from the handler map

The fallback overload could not infer `R` from the handlers: `R` appeared
only inside the `Exact` constraint, which is not an inference site, so with
inferred handler params it collapsed to `unknown` and the matcher silently
returned `UnaryFn<T, unknown>`. Adding `Partial<Handlers<T, R>>` to the
handler parameter gives `R` an inference site, so the common return is
inferred from the handlers and the fallback alike.

Tests drop their explicit handler return annotations: real handlers are short
and unannotated, and the common type is now inferred from them.

src/primitive.ts

@@ -32,7 +32,7 @@ type Fallback<T extends string | number, Handled, R> = UnaryFn<
 interface MatcherStrict<T extends string | number> {
     <R>(handlers: Handlers<T, R>): UnaryFn<T, R>;
     <R, Handled extends Exact<Partial<Handlers<T, R>>, Handled>>(
-        handlers: Handled,
+        handlers: Handled & Partial<Handlers<T, R>>,
         fallback: Fallback<T, Handled, R>,
     ): UnaryFn<T, R>;
 }