Match Qwen head-chunked attention execution

src/engine/metal/qwen_mtplx/policy.rs

@@ -25,6 +25,10 @@ fn falsey(value: Option<&str>) -> bool {
     })
 }
 
+pub(super) fn sdpa_head_chunk_enabled(get: impl Fn(&'static str) -> Option<String>) -> bool {
+    !falsey(get("MTPLX_QWEN4_VERIFY_SDPA_HEAD_CHUNK").as_deref())
+}
+
 pub(super) fn sorted_gather_enabled(get: impl Fn(&'static str) -> Option<String>) -> bool {
     !falsey(get("MTPLX_MOE_SORTED_GATHER_KERNEL").as_deref())
         && !truthy(get("MTPLX_FORCE_GPU_FAMILY_FALLBACK").as_deref())