Invoke a function and stream the response (SSE or chunked).

Streaming counterpart of /functions/{id}/invoke. The response shape is selected from the request's Accept header:

• Accept: text/event-stream — Server-Sent Events. Each chunk the guest emits via the wasi:tensor/host.emit-chunk host function is rendered as one event: chunk frame. The stream terminates with an event: done frame on success or an event: error frame on guest failure / deadline-elapsed.
• Otherwise — Content-Type: application/octet-stream, Transfer-Encoding: chunked. Each guest chunk is forwarded verbatim as one HTTP chunk frame, followed by the same terminal event: done / event: error line so clients on either negotiation outcome detect end-of-stream uniformly.

v0.4 wiring (T34)

The route is wired end-to-end through tensor_wasm_wasi_gpu::StreamingContext. Guest emits land on a tokio::sync::mpsc::Receiver<Vec<u8>> the gateway drains into the response body. The cooperative-deadline path (T36) routes a deadline-elapsed signal into a final event: error with {"reason":"deadline_elapsed"} so SSE clients can distinguish it from a generic trap. See docs/STREAMING.md.