Canary session token skew

v140 of the cart service auto-promotes from canary to 100% at 12:00 after a clean 30-minute canary. By 12:25 support reports a steady trickle of users whose carts 'reset to empty' after adding items. Overall add-to-cart success is 99.95%; only ~0.3% of sessions are affected. The canary used per-request random routing (a request could hit canary or baseline independently). v140 changed the cart session-token format. Triage and explain why the canary passed, then mitigate.