Hydra S2S — concurrency limits + per-org rate metering

Hydra S2S sessions now route through a per-org concurrency feature in the billing service. This puts an explicit ceiling on simultaneous live voice sessions per org and meters usage for billing.

What changed:

New feature-waves-hydra-concurrency plan feature: caps the number of concurrent open WebSocket sessions to wss://api.smallest.ai/waves/v1/s2s per org.
Sessions opened beyond the cap receive an error event with code: "server_full" followed by close code 1013. Back off with jitter and retry.
Usage is metered against your billing entitlement; check the usage field on response.done for per-turn token counts.

Migration: no action — existing Hydra integrations keep working. If you hit server_full repeatedly, contact your account manager to lift the concurrency cap.