✨ Add primitive union pattern matchers

Curried matchers over string|number literal unions in four variants
(exhaustive/partial x strict/widened return inference), per the header
matrix in src/primitive.ts.

- Adds type-fest for Simplify/ValueOf.
- Deliberate oxlint escape hatches (as any dispatch) until a cast-free
  formulation lands; see the file comments.

src/primitive.ts

@@ -0,0 +1,61 @@
+import type { Simplify, ValueOf } from "type-fest";
+
+type UnaryFn<T, R> = (shape: T) => R;
+
+// ============================================================================
+// ✔️ Exhaustive
+// ❌ ReturnsStrict
+// ============================================================================
+type PatternPrimitiveUnion<R, T extends string | number> = {
+    [K in T]: UnaryFn<K, R>;
+};
+
+type PatternReturns<
+    P extends Record<string | number, UnaryFn<never, unknown>>,
+> = ReturnType<ValueOf<P>>;
+
+export const getPrimitiveUnionMatcherW: <T extends string | number>() => <
+    P extends PatternPrimitiveUnion<unknown, T>,
+>(
+    pattern: Simplify<P>,
+) => UnaryFn<T, PatternReturns<P>> = () => (pattern) => (shape) =>
+    // oxlint-disable-next-line typescript/no-explicit-any typescript/no-unsafe-type-assertion
+    (pattern[shape] as any)(shape);
+
+// ============================================================================
+// ✔️ Exhaustive
+// ✔️ ReturnsStrict
+// ============================================================================
+export const getPrimitiveUnionMatcher: <T extends string | number>() => <R>(
+    pattern: Simplify<PatternPrimitiveUnion<R, T>>,
+) => UnaryFn<T, R> = getPrimitiveUnionMatcherW;
+
+// ============================================================================
+// ❌ Exhaustive
+// ✔️ ReturnsStrict
+// ============================================================================
+type PatternPrimitiveUnionPartial<R, T extends string | number> =
+    | PatternPrimitiveUnion<R, T>
+    | (Partial<PatternPrimitiveUnion<R, T>> & {
+          _: UnaryFn<T, R>;
+      });
+
+export const getPrimitiveUnionMatcherPartial: <T extends string | number>() => <
+    R,
+>(
+    pattern: Simplify<PatternPrimitiveUnionPartial<R, T>>,
+) => UnaryFn<T, R> = () => (pattern) => (shape) =>
+    // oxlint-disable-next-line typescript/no-explicit-any typescript/no-unsafe-type-assertion
+    (pattern[shape] ?? (pattern as any)["_"])(shape);
+
+// ============================================================================
+// ❌ Exhaustive
+// ❌ ReturnsStrict
+// ============================================================================
+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;