[codex] Allow exact standard stableswap bounds

[caydyan]

Summary

This PR lets the standard stableswap.clar accept exact user-provided lower bounds instead of requiring outputs to be strictly greater than the bound.

Changed standard pool checks from > to >= for:

• swap-x-for-y min-y-amount
• swap-y-for-x min-x-amount
• add-liquidity min-lp-amount
• withdraw-liquidity min-x-amount and min-y-amount

Because the withdraw bound now accepts equality, I added an explicit lp-amount > 0 guard so a zero-LP no-op cannot pass with zero minimums.

Why

A minimum output/slippage bound normally means "at least this amount." The previous strict comparisons rejected successful swaps, mints, and withdrawals when the realized amount matched the user bound exactly.

Validation

• git diff --check
• clarinet check (passes for all 12 contracts; existing warnings only)
• Manual clarinet console replay for exact-bound swap-x-for-y, exact-bound swap-y-for-x, exact-bound add-liquidity, and exact-bound withdraw-liquidity

Notes

I kept this scoped to the standard stableswap contract. The StackingDAO variant is covered separately in #18.