♻️ Rename primitive to primitive-union

`getMatcher` / `getMatcherW` become `getPrimitiveUnionMatcher` /
`getPrimitiveUnionMatcherW`; `src/primitive.ts` and its test move to
`src/primitive-union.*`. The `Union` suffix mirrors `getTaggedUnionMatcher`.

src/tagged-union.ts

@@ -36,7 +36,7 @@ type Handlers<T extends object, K extends keyof T, R> = {
 // The members `Handled` covers. Mapping over `Tags` keeps every index within
 // `MapTaggedUnion`'s keys, and the conditional drops a stray key outside `T` so
 // it cannot widen the remainder. The remainder is `Exclude<T, …>`, mirroring
-// the primitive matcher's `Exclude<T, PatternParam<keyof Handled>>`.
+// the primitive-union matcher's `Exclude<T, PatternParam<keyof Handled>>`.
 type HandledMembers<T extends object, K extends keyof T, Handled> = {
     [V in Tags<T, K>]: V extends keyof Handled
         ? MapTaggedUnion<T, K>[V]
@@ -52,7 +52,7 @@ type Fallback<T extends object, K extends keyof T, Handled, R> = UnaryFn<
 
 // A fallback is redundant once the handler map covers every tag of `T`. Folded
 // into `Handled`'s own (self-referential) constraint so it is checked *after*
-// inference; see the primitive matcher for why a conditional in the fallback's
+// inference; see the primitive-union matcher for why a conditional in the fallback's
 // parameter is evaluated too early.
 type MustBePartial<T extends object, K extends keyof T, Handled> =
     Tags<T, K> extends keyof Handled ? RedundantFallback : unknown;