test(comprehensive): reword the AuthBasic code-record invariant comment

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
E2E: sandbox, production

Author: belaboe

test-apps/comprehensive/test/basic-auth.test.ts

@@ -503,7 +503,8 @@ export function basicAuthTests(getApi: () => typeof apiType) {
         await api.authSignUp(username, 'password123');
         await pollForCode(api, username);
 
-        // Exactly one key. An unkeyed "latest code" slot is what made this
+        // Exactly one key for this user, and no record keyed by anything but
+        // its own username. An unkeyed "latest code" slot is what made this
         // flaky in the first place: whoever delivered last owned it, so a
         // reader could be handed another user's code. Nothing reads one, so
         // nothing writes one — and that also halves the writes per delivery.