♻️ 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/library.md

@@ -3,8 +3,8 @@
 The type-level design of the public API and the limitations it carries. The
 user-facing reference is [README § API](../README.md#api).
 
-The matchers are implemented in `src/primitive.ts` (`getMatcher` /
-`getMatcherW`) and `src/tagged-union.ts` (`getTaggedUnionMatcher` /
+The matchers are implemented in `src/primitive-union.ts` (`getPrimitiveUnionMatcher` /
+`getPrimitiveUnionMatcherW`) and `src/tagged-union.ts` (`getTaggedUnionMatcher` /
 `getTaggedUnionMatcherW`), re-exported from `src/index.ts`; the rest of the
 library is placeholder code.
 
@@ -16,8 +16,8 @@ A factory takes the universe and returns a builder; the builder takes a handler
 map and an optional fallback:
 
 ```ts
-const matcher = getMatcher<"a" | "b">()({ a: (s) => …, b: (s) => … });
-const fallback = getMatcher<"a" | "b" | "c">()({ a: (s) => … }, (s) => …);
+const matcher = getPrimitiveUnionMatcher<"a" | "b">()({ a: (s) => …, b: (s) => … });
+const fallback = getPrimitiveUnionMatcher<"a" | "b" | "c">()({ a: (s) => … }, (s) => …);
 ```
 
 Exhaustive or fallback is decided **at the call site**, by whether the second
@@ -25,8 +25,8 @@ argument is present. The fallback's parameter is the remainder
 `Exclude<T, keyof Handled>`. Only the return-strictness axis remains, so there
 are two factories:
 
-- `getMatcher` — one common `R`; the fallback must fit it;
-- `getMatcherW` — the union `PatternReturns<Handled> | R`.
+- `getPrimitiveUnionMatcher` — one common `R`; the fallback must fit it;
+- `getPrimitiveUnionMatcherW` — the union `PatternReturns<Handled> | R`.
 
 Each factory is two overloads whose order is load-bearing:
 
@@ -118,7 +118,7 @@ use: `UnaryFn`, `PatternReturns`, `RedundantFallback`, `HandlerMap`.
 
 #### Decision (2026-09)
 
-`getTaggedUnionMatcher` / `getTaggedUnionMatcherW` mirror the primitive pair
+`getTaggedUnionMatcher` / `getTaggedUnionMatcherW` mirror the primitive-union pair
 with one extra curried step for the discriminant key:
 
 ```ts
@@ -142,14 +142,14 @@ The key is a separate call because `K` is inferred from its literal argument and
 
 #### Why
 
-- **Same fallback/remainder machinery as the primitive matcher.** `HandledMembers`
+- **Same fallback/remainder machinery as the primitive-union matcher.** `HandledMembers`
   maps the handled tags to their members and `Exclude<T, …>` is the fallback's
   parameter; the redundant-fallback guard is the same F-bounded constraint. Only
   the "universe" changes — `T`'s members instead of primitive values.
 - **`T extends object`, not `Record<PropertyKey, unknown>`.** An `interface` has
   no implicit index signature, so the `Record` constraint would reject
   interface-based unions. The runtime reads the tag off `object` with one
-  assertion, the tagged twin of the primitive dispatch's `shape as string | number`.
+  assertion, the tagged twin of the primitive-union dispatch's `shape as string | number`.
 - **`MapTaggedUnion` distributes with `Extract`.** A duplicated tag yields a
   union of members instead of dropping one.
 
@@ -158,7 +158,7 @@ The key is a separate call because `K` is inferred from its literal argument and
 - Tags are `string | number` only. A `boolean` / `null` / `undefined`
   discriminant (`{ ok: true } | { ok: false }`) is rejected by `Discriminated`,
   because those values are not property keys; supporting them needs the
-  primitive matcher's `PatternKey` / `PatternParam` projection.
+  primitive-union matcher's `PatternKey` / `PatternParam` projection.
 - A member's tag must be unique across the union; two members with the same tag
   collapse to a union under one handler.
 

development/testing.md

@@ -28,7 +28,7 @@ before the implementation.
 - Runtime-first (classic red/green): it verifies the value, not the contract,
   and the contract is the product.
 - Testing the type only: it would not catch handler dispatch or the `_`
-  fallback (see `src/primitive.test.ts`).
+  fallback (see `src/primitive-union.test.ts`).
 
 The runner is `node --test --strip-types "src/**/*.test.ts"` and the tiers are
 in
@@ -167,7 +167,7 @@ The script prints the labels the server offers at a `/*COMPLETE*/` marker inside
 `<file>` (the marker is stripped before the document is sent). Its `LspSession`
 is imported by `src/util/__tests__/lsp-completion.test.ts` — which tests the
 helper itself against inline documents, never the library's code — and by
-`src/primitive.test.ts`, where the same probe asserts the matcher's popup;
+`src/primitive-union.test.ts`, where the same probe asserts the matcher's popup;
 the CLI is for manual inspection.
 
 #### Why

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