Function process_instruction 

pub fn process_instruction(
    program_id: &Pubkey,
    accounts: &[AccountInfo],
    instruction_data: &[u8],
) -> ProgramResult

Program entrypoint

Instruction discriminators:

• 0: create_escrow - Create a new payment escrow
• 1: confirm_delivery - Oracle confirms delivery, releases funds
• 2: refund_escrow - Refund after timeout
• 3: deposit_credits - Add prepaid credits
• 4: spend_credits - Oracle deducts credits for mail order
• 5: withdraw_credits - User withdraws unused credits