♻️ 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`.

development/tooling.md

@@ -107,7 +107,7 @@ Type-aware oxlint is enabled via `options.typeAware: true` in `.oxlintrc.json`
 #### Decision (2026-09)
 
 A type-aware rule that false-positives **at one site** is silenced with a
-source-level `oxlint-disable` directive (see `src/primitive.ts`). A rule that is
+source-level `oxlint-disable` directive (see `src/primitive-union.ts`). A rule that is
 wrong for a whole **file class** is turned off in a `.oxlintrc.json` `overrides`
 entry instead — e.g. `typescript/no-floating-promises` (synchronous
 `expectTypeOf` reads as an unhandled promise) and `unicorn/no-null` (intentional