fix(neuron): TP-vision Stage 0 — reject image requests on the TP path

The TP inference path has no vision tower, and the TP dispatch in
chat_completion / inference_stream returns before the VisionUnsupported
guard runs — so an image request to a TP-loaded model (e.g. beast's
tp=2 Qwen3.6-27B) was silently dropped and answered from text alone,
the exact issue-#3 confident-hallucination pattern Stage C killed for
single-GPU.

Add the request_has_images → VisionUnsupported guard to both
chat_completion_tp and inference_tp_stream, before prefill / before the
SSE stream opens, so beast returns a clean 400 vision_unsupported. The
guard is unconditional for now (TP has no tower); Stage 3 makes it
conditional on the TP model's has_vision once real TP-vision lands.

Detection is covered by the existing request_has_images unit test; the
guard itself is cuda-gated (validated by CI's CUDA type-check).

Refs TP-vision plan Stage 0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.claude/scheduled_tasks.lock

@@ -0,0 +1 @@
+{"sessionId":"a27586bb-2ca7-4e92-8d82-12f41b39f392","pid":3106893,"procStart":"59753850","acquiredAt":1780571089798}

crates/neuron/src/harness/candle.rs

@@ -2739,6 +2739,18 @@ impl CandleHarness {
             return Err(poisoned_error(&model_id));
         }
 
+        // Stage 0 (TP-vision): the TP path has no vision tower yet, so
+        // an image-bearing request can't be honoured. Reject it cleanly
+        // with `vision_unsupported` instead of silently dropping the
+        // image and answering from text alone (the issue-#3 confident-
+        // hallucination pattern). Made conditional on the TP model's
+        // `has_vision` once Stage 3 wires real TP-vision.
+        if request_has_images(&request) {
+            let _g = span.enter();
+            tracing::warn!("TP chat_completion: rejecting image request, TP vision unsupported");
+            return Err(InferenceError::VisionUnsupported { model_id });
+        }
+
         let tp_for_marker = Arc::clone(&tp);
         let handle = tokio::spawn(chat_completion_tp_inner(tp, request).instrument(span.clone()));
         match handle.await {
@@ -2816,6 +2828,20 @@ impl CandleHarness {
             return Err(poisoned_error(&request.model));
         }
 
+        // Stage 0 (TP-vision): reject image requests on the TP streaming
+        // path before opening the SSE stream — the TP path has no vision
+        // tower yet, so honouring the image is impossible and silently
+        // dropping it would hallucinate. Returns a clean 400; made
+        // conditional on `has_vision` in Stage 3.
+        if request_has_images(&request) {
+            tracing::warn!(
+                "TP chat_completion (stream): rejecting image request, TP vision unsupported"
+            );
+            return Err(InferenceError::VisionUnsupported {
+                model_id: request.model.clone(),
+            });
+        }
+
         let prompt = build_prompt_for_request(tp.chat_template.as_deref(), &request);
         let encoding = tp
             .tokenizer