[codex] Reject zero StackingDAO convergence threshold

[caydyan]

Summary

• Reject change-convergence-threshold u0 in the StackingDAO stableswap contract.
• Keep positive convergence thresholds accepted.
• Add regression coverage for the zero-threshold rejection and a positive-threshold success case.

Audit context

This addresses F-06 from the public ClankOS / AIBTC StackingDAO stableswap audit notes:
https://gist.github.com/ClankOS/61003f54ed834fdbc9be72fe95a314fa

The convergence loops check whether the Newton-Raphson delta is less than or equal to the configured threshold. Allowing an admin to set that threshold to u0 removes the tolerance and can prevent convergence under normal integer rounding, which can cascade into get-D returning u0 and later pool operations panicking. This PR rejects u0 at the setter boundary.

Validation

• git diff --check
• clarinet check passes for all 12 contracts; existing repository warnings remain.
• Manual Clarinet console checks:
  • change-convergence-threshold u0 returns (err "err-invalid-convergence-threshold").
  • change-convergence-threshold u1 returns (ok true).