[codex] Fix standard stableswap quote fee scaling

[caydyan]

Summary

• calculate get-dx and get-dy total swap fees from scaled token amounts
• add a mixed-decimal regression test that checks get-dx against swap-y-for-x

Why

The read-only quote helpers compute AMM balances in scaled precision, but their total-fee inputs used the unscaled token amount. For mixed-decimal pairs this can make the quote path diverge from the executable swap path. In the USDA/sUSDT pool, get-dx overquoted u9999948020 before the fix while swap-y-for-x returned u9994998022 for the same u100000000 USDA input.

Validation

• git diff --check
• clarinet check
• clarinet console confirmed get-dx and swap-y-for-x now both return u9994998022 for the mixed-decimal scenario above

Note: the local environment does not have deno, so the TypeScript Clarinet test was added but not executed here.