Challenge: Process a Transfer Instruction

Simulate Solana's process_instruction pattern in pure Rust. You'll process a transfer by validating balances, deducting fees, and returning updated state.

Your Task

Implement process_transfer(sender_balance, recipient_balance, amount, fee) that:

1. Checks the sender can afford amount + fee
2. If not, returns Err("INSUFFICIENT_BALANCE")
3. If yes, computes new balances and returns Ok((new_sender, new_recipient))

Requirements:

• Sender pays both the transfer amount AND the fee
• Recipient receives only the amount (not the fee)
• The fee is burned (deducted from sender, not added anywhere)
• Use checked arithmetic to prevent overflow

This mirrors how Solana programs work:

• process_instruction receives account balances
• Validates preconditions
• Mutates balances
• Returns success or error