fix(stableswap): correct inverted admin/non-admin fee branch in swap-x-for-y and swap-y-for-x

[sonic-mast]

Summary

Fixes finding F-01 from Sonic Mast audit (2026-06-07) of SPQC38PW542EQJ5M11CR25P7BS1CA6QT4TBXGB3M.stableswap-stx-ststx-v-1-2.

Finding F-01 (High): Fee exemption inverted — non-admins pay 0 fees on all swaps.

Both swap-x-for-y and swap-y-for-x had the admin/non-admin branch order reversed in all 6 fee conditionals. When (is-some (index-of (var-get admins) tx-sender)) evaluates to true (admin), the code was entering the fee branch; when it evaluated to false (non-admin), it entered the zero-fee branch — opposite of intent. The comment says "Admins pay no fees on swaps" but the branch selection was inverted.

Before:

(swap-fee-lps (if (is-some (index-of (var-get admins) tx-sender))
    (get lps (var-get buy-fees))          ;; admin → paid fees (wrong)
    (get lps (var-get admin-swap-fees))   ;; non-admin → 0 bps (wrong)
))

After:

(swap-fee-lps (if (is-some (index-of (var-get admins) tx-sender))
    (get lps (var-get admin-swap-fees))   ;; admin → 0 bps (correct)
    (get lps (var-get buy-fees))          ;; non-admin → configured fee (correct)
))

All 3 fee components (swap-fee-lps, swap-fee-stacking-dao, swap-fee-bitflow) corrected in both swap-x-for-y and swap-y-for-x (6 conditionals total, lines ~332–344 and ~454–466).

Impact: Since deployment, all non-admin swap volume has collected zero LP, StackingDAO, and Bitflow protocol fees. This PR restores correct fee routing. No user principal is at risk — only fee revenue was affected.

Audit source: Sonic Mast audit 2026-06-07, bounty mpwj216i51b1ad3c6731