Package-level declarations

NMI as a third vault-pattern target (roadmap #12) — same demonstration as provider:peach: the stored-instrument card_id vault (VaultStore/VaultRoutes/VaultBackend, roadmap #7) is processor-agnostic, so a second/third real-world processor (NMI's own card_id/customer_vault_id terminology is where this pattern's name comes from) can reuse it unchanged. Supersedes the nmiStubConfig docs-only entry — :provider:nmi is now a working (mock) PaymentGateway, not just a catalog stub. Mirrors provider:cash's lightest-module shape: no Compose, no hosted-webview.