feat(neuron): OpenAI-compatible non-streaming chat completion

Stage 3 of the candle-native pivot. neuron now serves
POST /v1/chat/completions backed by candle's quantized_qwen3 forward
pass on a per-model serialised generation loop, returning the standard
OpenAI ChatCompletionResponse envelope.

Pipeline per request:
- Look up the LoadedModel by request.model (404 if absent).
- Apply the Qwen3 chat template across all messages.
- Tokenize, then spawn_blocking onto tokio's blocking pool to acquire
  the per-model arch lock and run prefill + greedy/temperature/top-p
  sampling via LogitsProcessor.
- Stop on <|im_end|>/<|endoftext|> EOS or max_tokens (finish_reason
  "stop" vs "length").
- Decode with skip_special_tokens=true, build OpenAI response with
  prompt/completion/total usage counts.

Supporting changes:
- HarnessRegistry now stores Arc<dyn Harness> and caches a typed
  Arc<CandleHarness> so inference routes bypass dyn-Trait dispatch.
- LoadedModel.arch becomes Arc<Mutex<ModelArch>> so the lock guard
  can be moved into spawn_blocking.
- NeuronState gains an Option<Arc<CandleHarness>> field for the new
  inference route.
- Typed InferenceError lets the handler map ModelNotLoaded → 404 and
  other failures → 500 without string-matching anyhow messages.
- stream=true returns 501 until Stage 4 wires up SSE.
- Two leftover mistral.rs string references in proxy.rs and cortex-cli
  (missed during the Stage 1 sweep) are corrected here.

Three new default-feature tests cover the no-candle 503, model-not-
loaded 404, and stream=true 501 paths. The cuda-integration test from
Stage 2 still covers real load/unload; a streaming-feature gated test
exercising actual generation will arrive with Stage 4.

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

crates/neuron/src/main.rs

@@ -53,6 +53,7 @@ async fn main() -> Result<()> {
     // inference_endpoint.
     let registry = HarnessRegistry::from_configs(&cfg.harnesses, &bind_url, &cfg.harness);
     discovery_result.harnesses = registry.names();
+    let candle = registry.candle();
 
     let health_cache = Arc::new(health::HealthCache::new());
     health_cache
@@ -68,6 +69,7 @@ async fn main() -> Result<()> {
         discovery: discovery_result,
         health_cache,
         registry: RwLock::new(registry),
+        candle,
     });
 
     let app = api::neuron_routes().with_state(state);