…3-moe-streaming-sse-v1)

## Summary

Discharges qwen3-moe-streaming-sse-v1.yaml v1.0.0 (landed in #1835).
Post-M32d (#1832), MoE per-token generation amortizes to ~100ms; this
contract codifies that `stream=true` on a qwen3_moe model emits SSE
events per-token instead of buffering the full completion.

## Changes

- `infer/qwen3_moe_generate.rs`: add `run_qwen3_moe_generate_streaming`
  — callback variant of `run_qwen3_moe_generate`. Mirrors the
  non-streaming function step-for-step, but invokes `on_token(u32) -> bool`
  after each decoded token (BEFORE the stop check, so the client sees
  every sampled token). Callback returning `false` short-circuits the
  loop for client disconnect handling.

- `api/cuda_chat_backend.rs`: in `try_qwen3_moe_backend`, branch on
  `request.stream`. If true, spin up an mpsc channel, run the streaming
  variant on a `spawn_blocking` worker, and route the channel through
  the dense path's `true_streaming_sse_response` helper. Non-streaming
  path unchanged.

- `api/openai_handlers.rs`: promote `true_streaming_sse_response` from
  `fn` to `pub(crate) fn` so the MoE backend can reuse the same SSE
  framing as the dense path. No behavior change.

- `tests/qwen3_moe_streaming_sse_v1.rs`: env-gated integration tests
  (`QWEN3_MOE_GGUF_PATH`, `#[ignore]`'d) discharging V1_001 + V1_003:
    * V1_001: streaming callback fires per-token, captured tokens
      equal the non-streaming greedy baseline.
    * V1_003: median inter-token gap < 500ms (≥2 tok/s floor, well
      below M32d's ~5 tok/s).
    * Bonus: callback returning `false` short-circuits the loop.

V1_002 (`stream=false` regression) is covered by
`qwen3_moe_serve_dispatch_v1.rs`.

## Why

#1832 made KV cache available → per-token gen amortizes to ~100ms.
Before this PR, MoE `stream=true` requests on qwen3_moe still went
through `run_qwen3_moe_generate` (synchronous) and the client got the
full response in a single late SSE event — UX regression vs dense path.
Now the client sees the first token within `prefill_wall + 100ms` and
subsequent tokens stream at ~M32d throughput.

## Test plan
- [x] `cargo check -p aprender-serve --lib` — clean
- [x] `cargo test -p aprender-serve --lib qwen3_moe_generate` — 12/12 pass
- [ ] Operator-dispatched: `QWEN3_MOE_GGUF_PATH=… cargo test --test qwen3_moe_streaming_sse_v1 -- --ignored --nocapture` (env-gated, requires GGUF)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>