Spike proves response.body.tee() splits a real routerai SSE stream: the
consumer sees chunks progressively (first at 1ms of 1643ms) and our branch
still recovers the generation id and usage.cost. It also found a real
defect -- a rebuilt Response loses url, which the OpenAI SDK reads -- and
that Object.defineProperty is the only way to restore it.
Records what was verified against the live routerai API and the harness
code: the provider reports usage.cost on every response, the harness
discards it in mapUsage, and the cost can still be recovered by wrapping
globalThis.fetch while session attribution rides the llm/stream seam.
antonov