Port Qwen row-map SwiGLU expert gathers

src/engine/metal/qwen_mtplx/policy.rs

@@ -25,6 +25,11 @@ fn falsey(value: Option<&str>) -> bool {
     })
 }
 
+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())
+}
+
 fn strict_bool(name: &str, raw: Option<String>) -> Result<bool, String> {
     match raw
         .as_deref()
@@ -323,3 +328,19 @@ fn qwen_server_policy_matches_original_functions() {
     }
     assert_eq!(count, 268);
 }
+
+#[test]
+fn sorted_moe_switch_and_family_rehearsal() {
+    assert!(sorted_gather_enabled(|_| None));
+    for off in ["0", " false ", "NO", "off"] {
+        assert!(!sorted_gather_enabled(|key| (key
+            == "MTPLX_MOE_SORTED_GATHER_KERNEL")
+            .then(|| off.to_owned())));
+    }
+    for force in ["1", " true ", "YES", "on"] {
+        assert!(!sorted_gather_enabled(|key| (key
+            == "MTPLX_FORCE_GPU_FAMILY_FALLBACK")
+            .then(|| force.to_owned())));
+    }
+    assert!(sorted_gather_enabled(|_| Some("unexpected".into())));
+}