Challenge: Simulate a CPI Vault Transfer in Rust

Cross-Program Invocations (CPIs) let programs transfer SOL from PDA-controlled vaults. You'll simulate this pattern: derive a vault PDA, verify it has funds, and compute the transfer.

Your Task

Implement build_vault_transfer(user, vault_balance, recipient_balance, amount) that:

1. Derives the vault PDA using find_pda(&["vault", user], "system") to get the bump
2. Checks the vault has enough balance for the transfer
3. Returns Ok((new_vault_balance, new_recipient_balance, bump)) on success
4. Returns Err("INSUFFICIENT_VAULT_BALANCE") if vault can't cover the amount

This mirrors the real Anchor pattern:

invoke_signed(
    &transfer_instruction,
    &[vault_pda, recipient, system_program],
    &[&[b"vault", user.key().as_ref(), &[bump]]],
)?;

The bump is needed for invoke_signed — the program proves it controls the PDA.