refactor: extract StreamRelay and bound client delivery

ingestion and client emission currently share control flow, while the final client-facing channel is unbounded. bounding proposed worker queues alone would still allow slow-client buffering to grow. we should isolate emission and make backpressure reach the client boundary, independently of any worker-placement decision.

proposed second slice of #241, building on the synchronous ingestion boundary in #243. this is a suggested scope; feel free to revise, combine, or remove it.

we should be able to verify that:

• StreamRelay owns upstream/local emission, deferred-frame ordering, and sequence numbering, while normalization and item lifecycle state stay in the synchronous pipeline.
• upstream response-ID rewriting and tool-name restoration are not applied to already-correct local events; deferred output preserves the gateway-executed tool ordering contract and contiguous sequence numbers.
• client delivery and deferred buffers have explicit entry/byte limits, including handling for a single oversized frame. document retained response-state memory separately rather than claiming queue bounds cap the whole request.
• slow consumers apply backpressure without unbounded spillover or silently dropped semantic events; client disconnect and producer failure release resources and propagate errors without hanging.
• tests exercise deferred/index-less ordering, mixed upstream/local events, tiny queue capacities, oversized frames, slow clients, and cancellation. expose queue occupancy and buffered-byte measurements to tune limits.

the existing client channel is part of this boundary. no spawn_blocking migration is required for this slice.