[codex] Fix StackingDAO admin swap fees

[caydyan]

Summary

Fixes the StackingDAO stableswap fee selection for admin vs non-admin swaps.

swap-x-for-y and swap-y-for-x both documented that admins should use the admin swap fee schedule, but the branch was inverted: admins were charged the regular buy/sell fees while non-admin users received the admin fee schedule. With the default admin-swap-fees set to zero, this meant ordinary users could swap without the intended normal fees.

This updates both swap paths so:

• admins use admin-swap-fees
• non-admin users use buy-fees / sell-fees
• regression coverage checks non-admin swaps in both directions

This corresponds to the published stSTX/STX stableswap audit finding about the inverted fee branch.

Validation

• clarinet check passes (12 contracts checked; existing repository warnings remain)
• git diff --check
• Clarinet console spot checks:
  • admin swap-x-for-y small amount: (ok u10000) with default zero admin fee
  • non-admin swap-x-for-y small amount: (ok u9995) with 3/1/1 normal fees
  • admin swap-y-for-x small amount: (ok u10000) with default zero admin fee
  • non-admin swap-y-for-x small amount: (ok u9995) with 3/1/1 normal fees

Note: this repo's test files use the older Clarinet/Deno test style and this checkout does not include a package.json; current Clarinet 3 provides clarinet check but not the old clarinet test subcommand.