[codex] Reject zero standard stableswap convergence threshold

[caydyan]

Summary

• reject change-convergence-threshold u0 in the standard stableswap contract
• keep positive threshold updates unchanged
• add regression coverage for the rejected zero path and accepted positive path

Why

A zero convergence threshold removes the tolerance used by the iterative stable-swap solvers, which can make convergence behavior brittle or unreachable. The StackingDAO stableswap path now rejects this value; the standard stableswap path should apply the same guard.

Validation

• git diff --check
• clarinet check
• clarinet console confirmed (err "err-invalid-convergence-threshold") for u0 and (ok true) for u1

Note: the local environment does not have deno, so the TypeScript Clarinet tests were added but not executed here.