✨ 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.

backlog.tasks

@@ -8,10 +8,10 @@ Setup:
 ☐ Split off template into separate package => pi --session 01a07dde-7050-7054-bb36-1606d7eb2bc3 @low
 
 v1.0:
-☐ API surface is stable and fully typed
-    ☐ Finalize public exports in `src/index.ts`
-    ☐ Document all exported types and functions
-    ☐ Add JSDoc for public APIs
+✔ API surface is stable and fully typed @done
+    ✔ Finalize public exports in `src/index.ts` @done
+    ✔ Document all exported types and functions @done
+    ✔ Add JSDoc for public APIs @done
 ✔ Test coverage meets threshold @done
     ✔ Achieve 100% branch coverage on `src/primitive-union.ts` @done
     ✔ Achieve 100% branch coverage on `src/index.ts` @done