Checkout Returned

The buyer's browser is back from Stripe (the page's <CommerceReturn/> calls this once). Body: {outcome: "success" | "cancel"} — which of the two return URLs it landed on; "cancel" is also Stripe's "back to merchant" arrow. Records funnel.checkout.returned{outcome} and, for a cancel, funnel.checkout.cancelled. Changes no checkout state (the webhook stays the source of truth for payment); once per ref and outcome. Only the buyer's own checkout (else 404, like the status).