Function scale_amount 

pub fn scale_amount(
    amount: u64,
    from_decimals: u8,
    to_decimals: u8,
) -> Result<u64, ProgramError>

Scale a token amount between different decimal precisions (floor).

Converts amount denominated in from_decimals to the equivalent value in to_decimals. Uses u128 intermediate to prevent overflow.

let scaled = scale_amount(1_000_000, 6, 9)?; // USDC → SOL precision
assert_eq!(scaled, 1_000_000_000);