Phase 3a red: auth.unwrapAuth tests and stub

Tests for the password-only decryption chain that follows a successful
login (SRP or email OTP, with or without 2FA). The unwrap covers:
  password -> KEK (Argon2id) -> masterKey (secretbox) ->
  secretKey (secretbox) -> tokenBytes (sealed box) -> base64url token

Each test builds a synthetic AuthorizationResponse using libsodium
directly and asserts unwrapAuth recovers the inputs byte for byte. The
test file also functions as the canonical description of the protocol.

Adds src/auth/types.ts with KeyAttributes, SRPAttributes,
AuthorizationResponse, and LoginChallenge declarations matching the
README's API reference. src/auth/unwrap.ts is the throwing stub; the
real implementation lands next.

src/auth/unwrap.ts

@@ -0,0 +1,17 @@
+// Stub: see the README "Development workflow" section for TDD policy.
+
+import type { AuthorizationResponse } from "./types.js";
+
+export interface UnwrapResult {
+    masterKey: Uint8Array;
+    secretKey: Uint8Array;
+    publicKey: Uint8Array;
+    token: string;
+}
+
+export const unwrapAuth = async (
+    _response: AuthorizationResponse,
+    _password: string,
+): Promise<UnwrapResult> => {
+    throw new Error("auth.unwrapAuth not implemented");
+};