✨ Allow boolean and nullish tags in tagged unions

A `boolean` / `null` / `undefined` discriminant cannot key a handler
map, so the tagged-union matcher now routes it through the
`PatternKey` / `PatternParam` projection the primitive-union matcher
already used. Both matchers share the projection from
`matcher-shared.ts`; `MapTaggedUnion`, `Handlers`, `HandledMembers`
and `MustBePartial` key off the projected form and invert it to
recover the real member.

Covers exhaustive and fallback dispatch, the redundant-fallback
guard, and the LSP popup offering the tags by name.

src/matcher-shared.ts

@@ -8,6 +8,32 @@ import type { ValueOf } from "type-fest";
 // A handler: one universe member in, one return value out.
 export type UnaryFn<T, R> = (shape: T) => R;
 
+// `boolean`, `null` and `undefined` cannot be property keys, so a mapped type
+// over a universe that includes one keys each such member by its
+// stringification. `PatternParam` inverts that projection, so a handler callback
+// still receives the *real* member (`true`, not `"true"`). Both matchers use the
+// projection: the primitive-union matcher over its universe, the tagged-union
+// matcher over a discriminant property's values. See README § Caveats for the
+// limits.
+export type PatternKey<T> = T extends boolean
+    ? T extends true
+        ? "true"
+        : "false"
+    : T extends null
+      ? "null"
+      : T extends undefined
+        ? "undefined"
+        : T;
+export type PatternParam<K> = K extends "true"
+    ? true
+    : K extends "false"
+      ? false
+      : K extends "null"
+        ? null
+        : K extends "undefined"
+          ? undefined
+          : K;
+
 // `Extract` drops optional handlers (`undefined`) so `PatternReturns` also works
 // when `P`'s constraint has optional keys.
 export type PatternReturns<P> = ReturnType<