the platform-agnostic layer every cart adapter sits on, extracted so
HikaShop, VirtueMart, WHMCS and others share one engine instead of each
carrying a copy of the money math and the settle loop.

- Gateway: per-order subaddress derivation, fiat->XMR with a pluggable
  rate source (CoinGecko default, fixed rate, or a custom URL), the
  monero: URI, and the HMAC poll token.
- OrderStore: the storage contract an adapter implements over its cart's
  order API; ArrayOrderStore is the in-memory reference/test double.
- Settler: the poll -> detect -> settle loop. fails closed -- an empty or
  zero expected amount never settles, and a settle requires a real
  on-chain txid.
- pay_card: the customer-facing payment card, one canonical copy vendored
  into each adapter at build time.

verification stays view-key only: no monero-wallet-rpc, no daemon. gateway
and settler suites run under plain php (no composer).