[codex] Allow exact StackingDAO slippage bounds

[caydyan]

Summary

• Allow StackingDAO stableswap slippage guards to pass when the computed amount exactly equals the user-provided minimum.
• Update the affected swap, add-liquidity, and withdraw-liquidity checks from strict > to >=.
• Add exact-boundary coverage for swap-x-for-y, swap-y-for-x, add-liquidity, and withdraw-liquidity, while keeping failure tests above the actual output amount.

Audit context

This addresses F-05 from the public ClankOS / AIBTC audit notes:
https://gist.github.com/ClankOS/61003f54ed834fdbc9be72fe95a314fa

The prior strict comparisons rejected valid transactions where the caller's minimum output/LP/withdrawal bound matched the computed amount exactly. Slippage minimums are conventionally inclusive: the transaction should fail below the bound, not at the bound.

Validation

• git diff --check
• clarinet check passes for all 12 contracts; existing repository warnings remain.
• Manual Clarinet console exact-boundary checks:
  • swap-x-for-y with min-y = 9995 returns (ok u9995).
  • swap-y-for-x with min-x = 9995 returns (ok u9995).
  • add-liquidity with min-lp = 9998500 returns (ok u9998500).
  • withdraw-liquidity with exact min token amounts returns (ok { withdrawal-x-balance: u5000000000000, withdrawal-y-balance: u5000000000000 }).