feat(ios): document and enforce #if entry point pattern for Swift DCE

[kirich1409]

Summary

Update the iOS Swift wrapper and documentation to establish the #if compilation condition pattern for guarding Swift entry points under @LocalFlag flags.

Pattern to document

// Entry point guarded by @LocalFlag new_checkout (defaultValue = false)
#if !DISABLE_NEW_CHECKOUT
NewCheckoutButton()
#endif

// Deeplink handler
#if !DISABLE_NEW_CHECKOUT
case .newCheckout: NewCheckoutCoordinator.start()
#endif

Acceptance criteria

• README section: iOS integration guide with xcconfig setup steps
• FeatureFlags.swift updated with guidance comment linking to docs
• iosApp sample updated to demonstrate the pattern
• xcconfig file path convention documented (iosApp/Config/FeatureFlags.generated.xcconfig)