feat(arch): the SVE2.1 narrowing two-to-one family

Assisted-by: GLM 5.3 Flash

asm/extension_test.go

@@ -273,6 +273,15 @@ func TestExtensionNamesARM64(t *testing.T) {
 		"ZZIP2",
 		"ZZIPQ1",
 		"ZZIPQ2",
+		// The stage-four families: the SVE2.1 narrowing two-to-one set.
+		"ZADDHNB",
+		"ZADDHNT",
+		"ZRADDHNB",
+		"ZRADDHNT",
+		"ZRSUBHNB",
+		"ZRSUBHNT",
+		"ZSUBHNB",
+		"ZSUBHNT",
 		// The stage-three families: the SVE2 crypto group, the predicate
 		// counters and loop terminators with the 32-bit while compares, and
 		// the reductions into a SIMD register.
@@ -385,7 +394,7 @@ func TestExtensionNamesARM64(t *testing.T) {
 	if strings.Join(got, ",") != strings.Join(want, ",") {
 		t.Errorf("ExtensionNames(ARM64) = %v, want %v", got, want)
 	}
-	if n := len(arch.Extensions(arch.ARM64)); n != 381 {
-		t.Errorf("the family registers %d instructions, want 381", n)
+	if n := len(arch.Extensions(arch.ARM64)); n != 389 {
+		t.Errorf("the family registers %d instructions, want 389", n)
 	}
 }