✨ Finalize and document the public API surface

Export the matcher builder types from the barrel and rename them off the
internal Strict/Widening suffixes, so the type a factory returns is
nameable: PrimitiveUnionMatcher/W, TaggedUnionMatcher/W, and the
tagged-union factory types. Add TSDoc to every public symbol — the
declarations carry it into the published package — and pin the exported
names in src/index.test.ts.

Document the API in README and record the surface decision (and the
rejected matcher-shared export) in development/library.md.

development/library.md

@@ -5,8 +5,41 @@ user-facing reference is [README § API](../README.md#api).
 
 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.
+`getTaggedUnionMatcherW`), re-exported from `src/index.ts`; the rest of `src/`
+is implementation detail.
+
+## Public surface
+
+#### Decision (2026-09)
+
+`src/index.ts` exports the four factories and the builder types they return:
+`PrimitiveUnionMatcher` / `PrimitiveUnionMatcherW`, `TaggedUnionMatcher` /
+`TaggedUnionMatcherW`, and `TaggedUnionMatcherFactory` /
+`TaggedUnionMatcherWFactory`. Every exported symbol carries TSDoc. The
+universe-agnostic pieces in `matcher-shared.ts` stay internal.
+
+#### Why
+
+- The builder interfaces are already the inferred return types (they appear in
+  the emitted `.d.ts`), so exporting them only makes them nameable — a consumer
+  can annotate a factory result without restating its structure.
+- The `Strict` / `Widening` suffixes were dropped for the public names because
+  the factory names already carry the `W` axis; the type and function names line
+  up (`getPrimitiveUnionMatcher` → `PrimitiveUnionMatcher`).
+- TSDoc travels into the emitted declarations, so editor hovers and the
+  published package document the API without a hand-written `.d.ts`.
+- `matcher-shared.ts` stays internal: its types are plumbing (`Handlers`,
+  `Fallback`, `UniverseGate`, …) whose shape follows the implementation, and the
+  matchers are the stable contract.
+
+#### Rejected
+
+- **Exporting the `matcher-shared.ts` vocabulary** (`Matchable`, `UnaryFn`,
+  `PatternKey`, `Member`, `PatternReturns`, …). They appear in the public
+  signatures, but a consumer never needs to name them; exporting them would
+  freeze internals as API.
+- **A hand-written `.d.ts` or a separate API document.** It would drift from the
+  implementation; TSDoc is generated from the source.
 
 ## Matcher shape