fix: disable corex_moe_topk_softmax by default — .so causes garbage output

Sub686: model outputs gibberish (chinese→zeros, japanese→spanish)
because topk kernel .so compiled on dev machine produces wrong results
on competition platform. Default to False, enable only with env var.

qwen3_6_scripts/qwen3_5.py

@@ -186,7 +186,7 @@ _USE_COREX_MOE_DIRECT_ROUTED = (
     and env_bool("BI100_MOE_COREX_DIRECT_ROUTED", False))
 _USE_COREX_MOE_TOPK_SOFTMAX = (
     _corex_moe_topk_softmax is not None
-    and env_bool("BI100_MOE_COREX_TOPK_SOFTMAX", True))
+    and env_bool("BI100_MOE_COREX_TOPK_SOFTMAX", False))
 _USE_FUSED_MOE_ACTIVATION = env_bool("BI100_MOE_FUSED_ACTIVATION", True)