feat: add PrimerBillingAddressForm and form-state providers (ACC-6921)

[OnurVar]

Summary

• Adds PrimerBillingAddressForm — controlled public component with country row + first/last name + address1/2 + postal/city + state fields.
• Adds useBillingAddressForm hook backed by a BillingAddressFormStateProvider so form state survives nav transitions.
• Refactors useCardForm to read from a matching CardFormStateProvider (same lifecycle reason).
• Exposes setBillingAddress on PrimerCheckoutContextValue; the form debounces writes through it.
• Integrates billing section into CardFormScreen: card form → divider → billing section → shared Pay.

Out of scope (follow-ups)

• Country selector screen lives in ACC-7226 (stacked on this branch). The row here navigates to the route, but the screen is mounted there.
• Per-field gating via checkoutModules — ACC-7224 (iOS bridge) + ACC-7225 (Android bridge) first.

Jira

https://primerapi.atlassian.net/browse/ACC-6921

Test plan

• `yarn typecheck` clean
• `yarn lint` exit 0
• `yarn test` — 104 tests pass (same baseline)
• iOS simulator: drop-in flow → card form shows billing section → fill fields → Pay enables when card + billing both valid
• Android emulator: same