refactor: 从 core 中抽出跨模块契约建立 interface/common 模块分层

引入 interface/（零实现声明层）与 common/（共享 ADT 操作层），把跨模块
契约从 core 实现里抽出来，依赖方向回到单向（base ← interface ← common ←
各实现层）。docs/module-layering.org 记录完整方案与判定标准。

interface/（纯类型与多态接口声明，零 .c）：
- types.h / event.h / backend.h 从 core 移入
- effect.h 新建（effect_t 从 plan.h 抽出）

common/（多实现层共用的自包含操作，.h + .c）：
- event.{h,c}：event_reset / event_cleanup
- listeners.{h,c}：listeners 维护（add / cleanup）；notify 留 core
- window.{h,c}：window_list + layer_props/metadata cleanup + window_classify_layer
- workspace.{h,c}：workspace_desc（从 wm_desc.h 拆出，改真实函数）

window 相关整理：
- window_layer_type_t 上浮 interface/types.h（common 的 classify 需要）
- window_classify_layer 移 common/window
- window_info_t 独立成 core/window_info.h（state/command 共享，不寄生）
- 删除 wm_desc.h，base/window_list 并入 common/window

全项目 include 路径同步更新

src/common/window.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include "interface/types.h"
+
+window_layer_type_t window_classify_layer(const window_layer_props_t *props);
+void window_layer_props_cleanup(window_layer_props_t *props);
+void window_metadata_cleanup(window_metadata_t *metadata);
+
+typedef struct window_list_t {
+  zdwm_window_id_t *windows;
+  size_t count;
+  size_t capacity;
+} window_list_t;
+
+void window_list_push(window_list_t *window_list, zdwm_window_id_t window_id);
+void window_list_reset(window_list_t *window_list);
+void window_list_cleanup(window_list_t *window_list);