feat: support promotional-code transaction lifecycle

[Kapusch]

Summary

• present the native StoreKit 2 offer-code redemption sheet
• expose verified current and unfinished transactions with transaction IDs and signed JWS payloads
• defer transaction finishing until the consuming app acknowledges backend verification

Impact

BloomFly can reconcile promotional redemptions reliably, including temporary backend failures and app restarts.

Validation

• native Swift wrapper built for device and both simulator architectures
• managed binding build passed
• sample iOS build passed
• NuGet pack and package-layout validation passed
• workflow YAML validation passed