feat: Add direct native shuffle execution optimization

common/src/main/scala/org/apache/comet/CometConf.scala

@@ -340,6 +340,20 @@ object CometConf extends ShimCometConf {
       .booleanConf
       .createWithDefault(true)
 
+  val COMET_SHUFFLE_DIRECT_NATIVE_ENABLED: ConfigEntry[Boolean] =
+    conf(s"$COMET_EXEC_CONFIG_PREFIX.shuffle.directNative.enabled")
+      .category(CATEGORY_SHUFFLE)
+      .doc(
+        "When enabled, the native shuffle writer will directly execute the child native plan " +
+          "instead of reading intermediate batches via JNI. This optimization avoids the " +
+          "JNI round-trip for native plans whose inputs are all native scans " +
+          "(CometNativeScanExec, CometIcebergNativeScanExec). Supports single and multi-source " +
+          "plans (e.g., joins over native scans). " +
+          "This is an experimental feature and is disabled by default.")
+      .internal()
+      .booleanConf
+      .createWithDefault(false)
+
   val COMET_SHUFFLE_DIRECT_READ_ENABLED: ConfigEntry[Boolean] =
     conf(s"$COMET_EXEC_CONFIG_PREFIX.shuffle.directRead.enabled")
       .category(CATEGORY_SHUFFLE)