feat(neuron): OpenAI Responses API + ci cuda-check runner label

Step 2 of the Responses rollout: native `/v1/responses` endpoint on
neuron that consumes the same InferenceEvent stream as
`/v1/chat/completions` but emits it as the Responses API's named
SSE event family. No gateway-side translation.

## Surface

- `cortex-core::responses` envelope types: `ResponsesRequest`,
  `ResponsesInput` (text | items), `ResponsesInputItem` (message |
  function_call | function_call_output | reasoning),
  `ResponsesContentPart` (input_text | input_image | output_text),
  `ResponsesResponse`, `ResponsesOutputItem`, `ResponsesUsage`. Plus
  a `events::*` constant module so the projector and the wire shape
  stay in sync without string-typos.

- `neuron::wire::openai_responses`:
  - `request_to_chat(req)` flattens Responses input + instructions
    into a `ChatCompletionRequest` the candle harness already
    understands. Text-only Parts collapse to a string; mixed
    text+image Parts go to chat's content-array shape; reasoning
    items drop; function_call / function_call_output round-trip
    via tool_calls / tool_call_id metadata so the surface is
    consistent for the day the harness emits tool calls.
  - `project_responses_stream(rx, meta)` reads InferenceEvents
    and emits the eight named events that compose a Responses
    stream: response.created → output_item.added → content_part.added
    → output_text.delta×N → output_text.done → content_part.done
    → output_item.done → response.completed. Synthesises start
    frames if the producer skips Start (poisoned model, early
    disconnect) so the stream stays coherent.
  - `build_response(meta, text, reason, usage)` for the
    non-streaming path.

- `CandleHarness::inference_stream(req)` extracted from
  `chat_completion_stream`, returning a typed `InferenceStream`
  (event receiver + id/created/model_id metadata). Both
  `chat_completion_stream` and the new `responses_stream` are now
  thin wrappers that pick their wire projection. TP path got the
  same treatment (`chat_completion_tp_stream` → `inference_tp_stream`).

- `POST /v1/responses` route on neuron. Non-streaming returns one
  buffered `ResponsesResponse`; streaming returns axum SSE with
  both event names and JSON data per frame (Responses, unlike
  chat completions, uses named `event:` lines). Reused
  `inference_error_response` helper hoisted out so the chat and
  responses handlers share the InferenceError → HTTP mapping.

## CI

Also bundles the `cuda-check` runner-label fix from feedback on
commit 1859777: `runs-on: rpm` doesn't ship the CUDA toolkit so
cudarc's nvcc-version build script blew up. Switched to
`runs-on: cuda-13.0` per the existing labels.

## Scope cuts (documented in the modules)

- `previous_response_id` rejected at translate time with 400
  (`code: chained_conversation_not_supported`) — stateful chained
  conversations need a persistence layer we haven't built.
- Reasoning items dropped (no Qwen3 `<think>` routing yet).
- Single output item per response (one `"message"` carrying text);
  `function_call` items reserved but not synthesised.
- Streaming events cover the core set; `response.in_progress`
  and the web_search / image_generation event families are
  out-of-scope.

22 new tests: 5 in cortex-core (envelope round-trips), 13 in
neuron::wire (request translator + projector + non-streaming
builder), 4 in neuron's tests/api.rs (route surface — 503 when no
candle, 400 on previous_response_id, 404 on missing model for
both stream and non-stream).

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

.gitea/workflows/ci.yml

@@ -97,12 +97,14 @@ jobs:
   # the category of bug where a refactor compiles fine under the
   # default feature set (which is what the `clippy` and `test` jobs
   # exercise) but fails inside a `#[cfg(feature = "cuda")]` block.
-  # Lives on `runs-on: rpm` because that's where nvcc / cudarc's
-  # build prerequisites are installed; the generic `rust` runner
-  # doesn't have them.
+  # `runs-on: cuda-13.0` selects the runner that ships nvcc /
+  # cudarc's build prerequisites. The generic `rust` and `rpm`
+  # runners don't have them (the previous label `rpm` was tried
+  # first and tripped cudarc's `nvcc --version` build script —
+  # see commit history).
   cuda-check:
     name: CUDA type-check
-    runs-on: rpm
+    runs-on: cuda-13.0
     steps:
       - uses: actions/checkout@v4
       - name: cargo check --features cuda (with retry)