Phase 3a green: implement auth.unwrapAuth

The implementation is exactly the decryption chain documented in the
test file: deriveKEK -> decryptBox(masterKey) -> decryptBox(secretKey)
-> decryptSealed(token) -> toBase64URL. Errors from the underlying
crypto primitives propagate; the only added validation is the up-front
check that the response actually contains both keyAttributes and
encryptedToken (caller bug if not).

Also re-exports the auth/unwrap and auth/types public surface from
src/index.ts.

All 38 tests pass; make check and make docker are green.

src/index.ts

@@ -1 +1,9 @@
 export const VERSION = "0.0.0";
+
+export { unwrapAuth, type UnwrapResult } from "./auth/unwrap.js";
+export type {
+    AuthorizationResponse,
+    KeyAttributes,
+    LoginChallenge,
+    SRPAttributes,
+} from "./auth/types.js";