fix: replace symlinks with real files for Docker build

ex_engine/kernels/kernels.h

@@ -1 +0,0 @@
-../xllm_kernels/kernels.h

ex_engine/kernels/kernels.h

@@ -0,0 +1,11 @@
+/* Auto-generated aggregation header for xllm::kernel namespace.
+ * Equivalent to CMake cc_library(NAME kernels HDRS param.h ops_api.h).
+ * 
+ * AST Layer 3: kernel dispatch interface
+ * Called by: xllm_layers/ (Layer 2)
+ * Calls:     xllm_kernels/ilu/ (Layer 4)
+ */
+#pragma once
+
+#include "param.h"
+#include "ops_api.h"