🐛 Type partial W matcher without an any cast

getPrimitiveUnionMatcherPartialW was declared with the intended
P-inferring signature but assigned via `as any`, hiding that the
declared parameter was not assignable to the implementation's. The
union in PatternPrimitiveUnionPartial makes the `_` arm demand
`_ ∈ keyof P`, which the exhaustive arm cannot prove.

Intersect the inference hook Simplify<P> with the implementation's
parameter shape, R pinned to PatternReturns<P>, so the assignment
type-checks while callers keep inferring P from the argument.

Add a spec for the exhaustive (no `_`) pattern.

src/primitive.ts

@@ -55,7 +55,13 @@ export const getPrimitiveUnionMatcherPartial: <T extends string | number>() => <
 export const getPrimitiveUnionMatcherPartialW: <
     T extends string | number,
 >() => <P extends PatternPrimitiveUnionPartial<unknown, T>>(
-    pattern: Simplify<P>,
-
-    // oxlint-disable-next-line typescript/no-explicit-any typescript/no-unsafe-type-assertion
-) => UnaryFn<T, PatternReturns<P>> = getPrimitiveUnionMatcherPartial as any;
+    /*
+    `Simplify<P>` is the inference hook: callers infer `P` from the argument.
+     the second half pins the impl parameter's `R` to `PatternReturns<P>`.
+     that makes the `= getPrimitiveUnionMatcherPartial` assignment type-check.
+     neither half works alone.
+     without the witness the union's `_` arm demands `_ ∈ keyof P`.
+     without `Simplify<P>` the parameter types do not compare.
+    */
+    pattern: Simplify<P> & PatternPrimitiveUnionPartial<PatternReturns<P>, T>,
+) => UnaryFn<T, PatternReturns<P>> = getPrimitiveUnionMatcherPartial;