feat(neuron): OpenAI-compatible SSE streaming chat completions

Stage 4 of the candle-native pivot. /v1/chat/completions now switches
to text/event-stream when the request sets stream: true, emitting one
chat.completion.chunk per generated token followed by the OpenAI
[DONE] terminator.

Pipeline:
- chat_completion_stream creates a bounded mpsc::channel<ChatCompletionChunk>(32),
  sends the leading role chunk, then spawns a blocking task that
  acquires the per-model arch lock and runs the streaming generation
  loop.
- run_inference_streaming tracks a cumulative decoded prefix so each
  chunk's delta.content is the substring added since the last chunk —
  safe across BPE byte-fallback boundaries that would otherwise split
  multi-byte UTF-8 chars.
- The blocking task aborts cleanly if blocking_send fails (client
  disconnected), so generation stops when the SSE consumer hangs up.
- Final chunk carries finish_reason ("stop" on EOS, "length" on
  max_tokens). The handler appends data: [DONE] after the channel
  closes.

The Stage 3 streaming 501 placeholder test is repurposed: with the
streaming path live, an unloaded model now hits the same 404 surface
as the non-streaming path (the model lookup happens first).

cortex-gateway's existing proxy is unchanged — it already forwards
SSE bytes verbatim from Phase 2 work, so the candle SSE format passes
through unmodified.

Neuron Cargo.toml gains futures + tokio-stream (both already in
workspace deps) for ReceiverStream and stream combinators.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/neuron/tests/api.rs

@@ -273,10 +273,11 @@ async fn test_chat_completions_model_not_loaded() {
     assert_eq!(resp.status(), 404);
 }
 
-/// `/v1/chat/completions` with `stream: true` returns 501 until Stage 4
-/// wires up SSE.
+/// `/v1/chat/completions` with `stream: true` returns 404 when the
+/// model isn't loaded — same surface as the non-streaming path. The
+/// streaming code only kicks in once the model lookup succeeds.
 #[tokio::test]
-async fn test_chat_completions_streaming_not_yet_implemented() {
+async fn test_chat_completions_streaming_model_not_loaded() {
     use cortex_core::harness::HarnessConfig;
     use neuron::config::HarnessSettings;
 
@@ -306,12 +307,12 @@ async fn test_chat_completions_streaming_not_yet_implemented() {
     let resp = reqwest::Client::new()
         .post(format!("{url}/v1/chat/completions"))
         .json(&json!({
-            "model": "anything",
+            "model": "definitely/not-loaded",
             "messages": [{"role": "user", "content": "hi"}],
             "stream": true
         }))
         .send()
         .await
         .unwrap();
-    assert_eq!(resp.status(), 501);
+    assert_eq!(resp.status(), 404);
 }