fix: select_best_gpu respects candidate order — first non-busy wins

Previously it picked the least-loaded GPU globally, ignoring priority order.
Now it tries candidates in order: MoE → VLM → Dense. Only falls back to
least-loaded when ALL candidates are busy.

router/router.py

@@ -119,7 +119,11 @@ def is_gpu_busy(model):
     return active >= max_c
 
 def select_best_gpu(candidates, reason):
-    """Pick the best GPU from candidates, preferring least-loaded."""
+    """Pick the best GPU from candidates IN ORDER — first non-busy one wins."""
+    for m in candidates:
+        if not is_gpu_busy(m):
+            return {"model": m, "reason": reason}
+    # All busy — pick least loaded
     best = None
     best_load = 999
     for m in candidates:
@@ -128,10 +132,7 @@ def select_best_gpu(candidates, reason):
             best_load = load
             best = m
     if best:
-        actual_reason = reason
-        if is_gpu_busy(best):
-            actual_reason = "load_balanced_" + reason
-        return {"model": best, "reason": actual_reason}
+        return {"model": best, "reason": "load_balanced_" + reason}
     return None
 
 def route(rd, tier):