[COMSE-27] Example app: Cash App Pay on-file vs one-time grant mode toggle

[noahjalex]

🚧 Draft / WIP — pushed to avoid losing work; not yet ready for review.

Linear: COMSE-27

Summary

Adds a One-time / On-file segmented control to the Example app's cart so the V3 Cash App Pay flow can request either PayKit grant type. Both modes hit the same Afterpay V3 CAP endpoints (POST /v3/button, /v2/payments/sign-payment, /v3/button/confirm); only the PayKit PaymentAction attached to the customer request differs:

• .oneTimePayment(scopeID:, money:) — current behavior, single-use authorization.
• .onFilePayment(scopeID:, accountReferenceID:) — grant suitable for on-file storage / merchant-initiated transactions.

This is purely Example-app plumbing for support / validation work — no changes to the public Afterpay SDK API.

Changes

• Example/Example/Purchase/ButtonCashAppPayCheckout.swift
  • New CashAppPayMode enum (.oneTime / .onFile).
  • checkoutV3(consumer:cartTotal:mode:) stores the selected mode and threads it through createCustomerRequest, which now branches the PaymentAction.
  • NSLog("[CAP-AP] …") traces across checkoutV3, customer-request creation, PayKit state changes, approval, and confirmCheckout.
• Example/Example/Purchase/CartViewController.swift
  • UISegmentedControl ("One-time" / "On-file") above the V3-CAP button; hidden whenever the V3-CAP button is hidden.
  • New event payload Event.didTapSingleUseCardButtonWithCashAppPay(CashAppPayMode).
• Example/Example/Purchase/PurchaseFlowController.swift / PurchaseLogicController.swift
  • Pass mode through to payWithAfterpayV3WithCashAppPay(mode:).

Out of scope

• Public SDK surface (Afterpay.checkoutV3WithCashAppPay / checkoutV3ConfirmForCashAppPay) is unchanged.
• Recurring/MIT capture flow (this PR only validates the grant; capture is server-side).

Test plan (not yet executed)

• Run Example app, switch checkout option to V3, toggle mode = One-time, confirm Cash App Pay flow completes (existing path).
• Toggle mode = On-file, confirm PayKit returns an approved customer request whose grant.action is onFilePayment and that /v3/button/confirm succeeds.
• Inspect [CAP-AP] logs for both modes.