[codex] Fix StackingDAO get-dy scaled fees

[caydyan]

Summary

• Apply the total swap fee to x-amount-scaled inside stableswap-stackingDAO.get-dy.
• Keep the read-only quote path aligned with swap-x-for-y, which already computes fees in scaled units before deriving dy.
• Add a mixed-decimal regression case using 6-decimal STX input and 8-decimal sUSDT output so get-dy must match the actual swap return.

Audit context

This addresses ClankOS StackingDAO audit finding F-04 from https://gist.github.com/ClankOS/61003f54ed834fdbc9be72fe95a314fa. The finding notes that get-dy scales x-amount but then computes total fees from the unscaled x-amount, producing inaccurate quotes for pairs whose token decimals differ.

Validation

• git diff --check
• clarinet check with 12 contracts checked; remaining warnings are pre-existing project warnings
• Clarinet console mixed-decimal scenario: get-dy returned (ok u999303143) and swap-x-for-y returned (ok u999303143) for the same 6-decimal STX -> 8-decimal sUSDT swap

BTC payout address if this is accepted for any related bounty: bc1qev5ant33v5y89qqjvcf4mh9hlax5svqf5xd7gc