Stable checkpoint — invariant 2 + smoke flow

Cumulative since stable-v1.2.5 :
- 5 jqwik property tests on Product invariant 2 (stock + price non-negativity)
- 8 hurl steps in smoke.hurl exercising Order/Product/OrderLine E2E
- Tests directly against Bean Validation + DB CHECK constraints

Post-merge main pipeline #2480773494 green at 20:12.