Attacker-only arming. First call sets the admin; later calls require it.
fn admin_set(
env: soroban_sdk::Env,
admin: soroban_sdk::Address,
sac: soroban_sdk::Address,
to: soroban_sdk::Address,
amt: i128,
)fn checkpoint(
env: soroban_sdk::Env,
user: soroban_sdk::Address,
val: soroban_sdk::Symbol,
hook: soroban_sdk::Address,
) -> u32