🐛 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.test.ts

@@ -22,11 +22,11 @@ test("getMatcher: exhaustive pattern infers one common return type", () => {
 
     // Act
     const matcher = factory({
-        a: (s): number => {
+        a: (s) => {
             expectTypeOf(s).toEqualTypeOf<"a">();
             return 1;
         },
-        b: (s): 1 | 2 => {
+        b: (s) => {
             expectTypeOf(s).toEqualTypeOf<"b">();
             return 2;
         },
@@ -69,19 +69,19 @@ test("getMatcher: dispatches on mixed string and numeric keys", () => {
 
     // Act
     const matcher = factory({
-        a: (s): number => {
+        a: (s) => {
             expectTypeOf(s).toEqualTypeOf<"a">();
             return 1;
         },
-        b: (s): 1 | 2 => {
+        b: (s) => {
             expectTypeOf(s).toEqualTypeOf<"b">();
             return 2;
         },
-        1: (n): number => {
+        1: (n) => {
             expectTypeOf(n).toEqualTypeOf<1>();
             return 10;
         },
-        2: (n): number => {
+        2: (n) => {
             expectTypeOf(n).toEqualTypeOf<2>();
             return 20;
         },
@@ -106,15 +106,15 @@ test("getMatcher: a `_` fallback receives the unhandled keys", () => {
     // Act
     const matcher = factory(
         {
-            a: (s): 1 | 2 => {
+            a: (s) => {
                 expectTypeOf(s).toEqualTypeOf<"a">();
-                return 1;
+                return 1 as const;
             },
         },
-        (s): 1 | 2 => {
+        (s) => {
             // The fallback sees only the keys `a` did not handle.
             expectTypeOf(s).toEqualTypeOf<"b" | "c">();
-            return 2;
+            return 2 as const;
         },
     );
 
@@ -134,16 +134,16 @@ test("getMatcher: a `_` fallback routes mixed string and numeric gaps", () => {
     // Act
     const matcher = factory(
         {
-            a: (s): string => {
+            a: (s) => {
                 expectTypeOf(s).toEqualTypeOf<"a">();
                 return "A";
             },
-            1: (n): string => {
+            1: (n) => {
                 expectTypeOf(n).toEqualTypeOf<1>();
                 return "one";
             },
         },
-        (s): string => {
+        (s) => {
             expectTypeOf(s).toEqualTypeOf<"b" | 2>();
             return "fallback";
         },