Check browser readiness for member Google sign-in

Read-only check of a ten-minute entry handle and the live signed OpenWork browser cookie, never a bearer substitute. Returns sign_in_required without a live cookie, account_mismatch for another signed-in user without revealing identities, or ready only after validating the original member, provider, credential set, OAuth client configuration and current grants. Does not consume or rotate the entry, create Google state, exchange tokens or revoke credentials. Browser-start and callback independently repeat authorization checks.