Rust fix 4

[johngrantuk]

Report issue:

4. compute_and_charge_aggregate_swap_fees returns raw units but reads as scaled-18

Impact: the crate is internally safe, but the API shape misleads consumers. It cost us a real
bug that a wei-exact amountOut parity suite could not catch.

The helper mirrors the Vault and returns the fee in the token's raw units:

let total_swap_fee_amount_raw = to_raw_undo_rate_round_down(swap_fee_amount_scaled18, sf, rate)?;
Ok(mul_down_fixed(&total_swap_fee_amount_raw, aggregate_swap_fee_percentage)?)

vault::swap::swap binds it to a scaled-18 name and subtracts it from scaled-18 balances:

let aggregate_swap_fee_amount_scaled_18 = compute_and_charge_aggregate_swap_fees(...)?;
// ...
updated_balances[input_index] += amount_given_scaled_18 - aggregate_swap_fee_amount_scaled_18;

Inside the crate this is harmless — updated_balances only feeds AfterSwapParams, and swap()
returns Result<U256, _>, so for hookless pools the value is computed and discarded.

It is a trap for anyone reproducing that balance update, which a quoter must do to report post-swap
state for multi-hop routing. We copied the pattern and under-deducted the protocol fee by the
token's scaling factor — 1e12 for USDC, and exactly zero for 18-decimal tokens at a rate of one,
which is why it survived our parity suite for so long. Measured with a 6-decimal input token, 1%
swap fee, 25% aggregate share:

expected deduction: 250000000000000000   (scaled-18)
actual   deduction:            250000    (raw)

Suggested fix: rename the function to ..._raw, or return a newtype carrying the unit. Renaming
the local in vault::swap::swap alone would remove most of the hazard.