Shadow Traffic, Dark Launches and Flag Releases

# Istio: mirror 100% of traffic to v2, serve from v1
http:
- route: [ { destination: { host: ledger, subset: v1 }, weight: 100 } ]
  mirror: { host: ledger, subset: v2 }
  mirrorPercentage: { value: 100.0 }

# in code: compare an old and new implementation on the same input (Scientist pattern)
const result = await experiment('fee-calc-v2', {
  control: () => feeV1(transfer),          // returned to the caller
  candidate: () => feeV2(transfer),        // run, compared, discarded
  compare: (a, b) => a.kobo === b.kobo,    // mismatches logged with inputs
});