Guard large unaligned Qwen expert gathers

src/engine/metal/qwen_mtplx/moe.rs

@@ -27,7 +27,7 @@ fn gather(
     streams: &Streams,
     stream: Stream,
 ) -> Result<Array, String> {
-    linear::gather_qmm_bf16(
+    linear::guarded_gather_qmm_bf16(
         input,
         [&layer.weight, &layer.scales, &layer.biases],
         [None, Some(ids)],