✨ 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.

CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- export the matcher builder types from the entry point —
+  `PrimitiveUnionMatcher` / `PrimitiveUnionMatcherW`, `TaggedUnionMatcher` /
+  `TaggedUnionMatcherW`, and the tagged-union factory types — and add TSDoc to
+  every public symbol
+- document the public API in the README
+
 ## [0.8.1] - 2026-09-23
 
 - gate CI at 100% coverage: `test:ci` runs c8 with `--all --100` over `src/`,