feat: Add entry points & Debug App

[bthebladeprimer]

Summary

• Add PrimerCheckout — the main public entry point for creating and presenting checkout flows
• Add PrimerCheckoutPresenter — handles presentation logic, coordinator setup, and scope lifecycle
• Add Debug App integration with CheckoutComponents demos: DefaultCheckoutDemo, CustomPaymentSelectionDemo
• Add Debug App infrastructure: CheckoutComponentsMenuViewController, CheckoutComponentsExamplesView, DemoRegistry, DemoProtocol
• Add Debug App shared components: DemoRow, ErrorView, LoadingView, NetworkingUtils, SimpleCountryPicker
• Update existing Debug App files (AppDelegate, SceneDelegate, storyboard, project files) for CheckoutComponents navigation
• Add API_REFERENCE.md and CLAUDE.md documentation for CheckoutComponents

Context

PR 15 of 16 in the CheckoutComponents feature split. Depends on PR 14 (UI layer).

Notion tracker

Test plan

• Build Debug App in Xcode and verify CheckoutComponents menu appears
• Run DefaultCheckoutDemo and verify end-to-end checkout flow
• Run CustomPaymentSelectionDemo and verify custom selection flow
• Verify PrimerCheckout public API initializes correctly
• Verify PrimerCheckoutPresenter manages scope lifecycle

[github-actions]

	Warnings
⚠️	This PR doesn't seem to contain any updated Unit Test 🤔. Please consider double checking it.🙏
⚠️	> Pull Request size seems relatively large. If this Pull Request contains multiple changes, please split each into separate PR will helps faster, easier review.

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