feat(core): 引入 plan/effect 执行骨架

新增 plan/effect 数据结构与基础操作
在 runtime 中接入 event -> command_buffer -> plan -> backend 流程
补齐 policy/backend 接口并同步更新测试构建

tests/core/CMakeLists.txt

@@ -10,9 +10,12 @@ add_executable(${TEST_APP_NAME}
     ${SOURCE_DIR}/backend/x11/backend.c
     ${SOURCE_DIR}/backend/x11/event.c
     ${SOURCE_DIR}/backend/x11/window.c
+    ${SOURCE_DIR}/core/command_buffer.c
     ${SOURCE_DIR}/core/event.c
     ${SOURCE_DIR}/core/layer.c
     ${SOURCE_DIR}/core/layout.c
+    ${SOURCE_DIR}/core/plan.c
+    ${SOURCE_DIR}/core/policy.c
     ${SOURCE_DIR}/core/rules.c
     ${SOURCE_DIR}/core/runtime.c
     ${SOURCE_DIR}/core/state.c