pet.trezy.swapPets

Change which of the caller's pets are on the stage. Pass `activate` to bring a pet on, `deactivate` to take one off, or both to swap in one call. Acts only on the authenticated caller's own rows and never accepts a DID. TOUCHES NO MONEY AT ALL: it writes a stint to trezy_pet_stage_log recording exactly when each pet went on or came off, and the accrual reads those stints so every pet is paid for the stretches it was genuinely on stage and no others. Coins already waiting stay waiting — nothing is collected, banked or forfeited by swapping, and the player picks them up whenever they like. A pet with no stints at all is treated as having been on stage since it was bought, which is what lets accounts that predate the log keep working unchanged.