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.c

@@ -0,0 +1,22 @@
+#include "core/plan.h"
+
+#include "base/array.h"
+#include "base/memory.h"
+
+void plan_reset(plan_t *plan) {
+  if (!plan->effects || !plan->capacity) return;
+
+  p_clear(plan->effects, plan->capacity);
+  plan->count = 0;
+}
+
+void plan_cleanup(plan_t *plan) {
+  p_delete(&plan->effects);
+  plan->count = 0;
+  plan->capacity = 0;
+}
+
+void plan_push_effect(plan_t *plan, const effect_t *effect) {
+  effect_t *eft = array_push(plan->effects, plan->count, plan->capacity);
+  *eft = *effect;
+}