attachSecureStore

Turn on per-send rollback protection: hold store so every send and every ratchet-advancing receive commits a monotonic counter to it, and exportStateSealed binds that counter. Call once, before sending, on a client made by connect / signIn; the sealed restore constructors attach the store themselves. This is what closes EX-03's per-send window — use the same platform store across restarts so the counter is monotone.