Extract LoyaltyStampCost and add LoyaltyDiscountPercent constant. Add first-payment guard to ApplyLoyaltyRedemption — reject if a non-discount payment already exists on the booking.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>