Boss build seeds one Fire Pass key for every agent. Three concurrent
agents -> three concurrent streamSimple calls against the Kimi K2.6
Turbo router -> three HTTP 429s on the overview dashboard.
Add an in-memory async semaphore keyed by (provider, sha1(apiKey)) and
acquire a slot inside `createSafePiStream` before invoking the source.
The slot is released when the stream finishes, errors, or aborts.
`fireworks-firepass` is capped at 1 concurrent call; every other
provider stays uncapped (no fallback layer, no 429 retry — flow control
prevents the burst at the source instead of recovering after).
Same code path applies to public and boss builds: only the seeded key
differs, the gate behaviour is identical.
ZacharyZhang-NY