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/base/window_list.c

@@ -1,23 +0,0 @@
-#include "base/window_list.h"
-
-#include <zdwm/types.h>
-
-#include "base/array.h"
-#include "base/memory.h"
-
-void window_list_push(window_list_t *window_list, zdwm_window_id_t window_id) {
-  zdwm_window_id_t *window =
-    array_push(window_list->windows, window_list->count, window_list->capacity);
-  *window = window_id;
-}
-
-void window_list_reset(window_list_t *window_list) {
-  p_clear(window_list->windows, window_list->capacity);
-  window_list->count = 0;
-}
-
-void window_list_cleanup(window_list_t *window_list) {
-  p_delete(&window_list->windows);
-  window_list->count = 0;
-  window_list->capacity = 0;
-}

src/base/window_list.h

@@ -1,14 +0,0 @@
-#pragma once
-
-#include <stddef.h>
-#include <zdwm/types.h>
-
-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);