…3 falsifiers PASS) (#1855)

* feat: qwen3-moe streaming SSE — per-token emit when stream=true (qwen3-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>

* evidence: qwen3-moe-streaming-sse-v1 DISCHARGED on gx10 Blackwell

## Summary

Operator-dispatched verification of `contracts/qwen3-moe-streaming-sse-v1.yaml`
v1.0.0 (shipped via #1854). All three falsifiers PASS on Qwen3-Coder-30B-A3B
(qwen3moe arch) running on gx10 Blackwell GB10.

## Results

| Falsifier   | Test                                      | Verdict |
|-------------|-------------------------------------------|---------|
| V1_001      | v1_001_callback_fires_per_token            | PASS    |
| V1_002      | (regression guard via serve-dispatch test) | GUARD   |
| V1_003      | v1_003_inter_token_latency_floor           | PASS    |

V1_003 throughput on real 30B-MoE: **median 338 ms inter-token gap** over
32 callbacks (floor 500 ms), distribution p_min=250 ms / p_max=518 ms.
≈ 3 tok/s streamed — comfortably above the 2 tok/s contract floor and
consistent with M32d's KV-cache-amortized per-token cost.

Plus the negative-path `callback_stop_short_circuits` test confirmed
that returning `false` from the per-token callback short-circuits the
decode loop (client-disconnect handling).

## Artifacts

- `findings.json` — machine-readable discharge record
- `gx10-sse-smoke.log` — full cargo test stdout/stderr (549 lines)

Both captured from `/home/noah/runs/sse-smoke-20260521-080640/` on gx10.

## Reproducer

```bash
QWEN3_MOE_GGUF_PATH=/path/to/Qwen3-Coder-30B-A3B-Instruct-Q4_K_M.gguf \
  cargo test --test qwen3_moe_streaming_sse_v1 \
    -p aprender-serve --features cuda --release \
    -- --ignored --nocapture
```

Binary commit: 6bff4ce (post-#1854 merge).

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

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

* evidence: add gx10 cargo test log (qwen3-moe-streaming-sse-v1 discharge)

Force-added (matches `.log` gitignore pattern but this one is a
load-bearing discharge artifact, not a temp file).

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>