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/bar/bar.c

@@ -20,7 +20,6 @@
 #include "base/macros.h"
 #include "base/memory.h"
 #include "base/time.h"
-#include "core/listeners.h"
 
 static zdwm_bar_item_t *bar_add_item(
   zdwm_output_id_t output_id,

src/bar/bar.h

@@ -10,7 +10,7 @@
 #include "bar/text.h"
 #include "bar/types.h"
 #include "base/color.h"
-#include "core/listeners.h"
+#include "common/listeners.h"
 
 typedef struct bar_output_t {
   cairo_t *cr;

src/bar/binding.c

@@ -9,7 +9,7 @@
 #include <zdwm/types.h>
 
 #include "base/memory.h"
-#include "core/listeners.h"
+#include "common/listeners.h"
 
 typedef struct bar_binding_state_t {
   zdwm_binding_mode_notify_t mode;

src/bar/binding.h

@@ -3,7 +3,7 @@
 #include <stdint.h>
 #include <zdwm/bar.h>
 
-#include "core/listeners.h"
+#include "common/listeners.h"
 
 typedef struct bar_binding_config_t {
   bool show_default;

src/bar/windows.c

@@ -8,7 +8,7 @@
 
 #include "base/array.h"
 #include "base/memory.h"
-#include "core/listeners.h"
+#include "common/listeners.h"
 
 typedef struct bar_windows_state_t {
   zdwm_output_id_t output_id;

src/bar/windows.h

@@ -3,7 +3,7 @@
 #include <stdint.h>
 #include <zdwm/bar.h>
 
-#include "core/listeners.h"
+#include "common/listeners.h"
 
 typedef struct bar_windows_config_t {
   uint32_t cell_padding;

src/bar/workspaces.c

@@ -10,7 +10,7 @@
 
 #include "base/array.h"
 #include "base/memory.h"
-#include "core/listeners.h"
+#include "common/listeners.h"
 
 typedef struct bar_workspace_t {
   zdwm_workspace_t info;

src/bar/workspaces.h

@@ -3,7 +3,7 @@
 #include <stdint.h>
 #include <zdwm/bar.h>
 
-#include "core/listeners.h"
+#include "common/listeners.h"
 
 typedef struct bar_workspace_config_t {
   uint32_t cell_padding;