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

src/core/plan.h

@@ -0,0 +1,44 @@
+#pragma once
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "core/types.h"
+
+typedef uint32_t dirty_mask_t;
+
+typedef enum dirty_flags_t : dirty_mask_t {
+  ZDWM_DIRTY_NONE = 0,
+  ZDWM_DIRTY_LAYOUT = 1u << 0,
+} dirty_flags_t;
+
+typedef enum effect_type_t {
+  ZDWM_EFFECT_NONE,
+  ZDWM_EFFECT_MAP_WINDOW,
+  ZDWM_EFFECT_UNMAP_WINDOW,
+  ZDWM_EFFECT_FOCUS_WINDOW,
+} effect_type_t;
+
+typedef struct effect_only_window_id_t {
+  window_id_t window;
+} effect_only_window_id_t;
+
+typedef struct effect_t {
+  effect_type_t type;
+  union {
+    effect_only_window_id_t map;
+    effect_only_window_id_t unmap;
+    effect_only_window_id_t focus;
+  } as;
+} effect_t;
+
+typedef struct plan_t {
+  effect_t *effects;
+  size_t count;
+  size_t capacity;
+  dirty_mask_t dirty_flags;
+} plan_t;
+
+void plan_reset(plan_t *plan);
+void plan_cleanup(plan_t *plan);
+void plan_push_effect(plan_t *plan, const effect_t *effect);