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/backend/output_utils.c

@@ -4,8 +4,8 @@
 #include <stdint.h>
 
 #include "base/memory.h"
-#include "core/backend.h"
-#include "core/types.h"
+#include "interface/backend.h"
+#include "interface/types.h"
 
 static inline int32_t right(rect_t a) { return a.x + a.width; }
 static inline int32_t bottom(rect_t a) { return a.y + a.height; }

src/backend/output_utils.h

@@ -2,8 +2,7 @@
 
 #include <stddef.h>
 
-#include "core/backend.h"
-#include "core/types.h"
+#include "interface/backend.h"
 
 backend_detect_t *
 output_remove_duplication(const output_info_t *output, const size_t count);

src/backend/x11/backend.c

@@ -1,4 +1,4 @@
-#include "core/backend.h"
+#include "interface/backend.h"
 
 #include <cairo-xcb.h>
 #include <cairo.h>
@@ -25,11 +25,7 @@
 #include "base/log.h"
 #include "base/macros.h"
 #include "base/memory.h"
-#include "base/window_list.h"
-#include "core/event.h"
-#include "core/plan.h"
-#include "core/types.h"
-#include "core/window.h"
+#include "common/window.h"
 #include "internal.h"
 
 typedef struct atom_item_t {

src/backend/x11/common.c

@@ -5,7 +5,7 @@
 #include <xcb/xproto.h>
 
 #include "base/macros.h"
-#include "core/types.h"
+#include "interface/types.h"
 
 typedef struct modifier_map_t {
   modifier_bit_t modifier;

src/backend/x11/common.h

@@ -3,7 +3,7 @@
 #include <stdint.h>
 #include <xcb/xproto.h>
 
-#include "core/types.h"
+#include "interface/types.h"
 
 modifier_mask_t modifiers_xcb_to_zdwm(uint16_t mask);
 uint16_t modifiers_zdwm_to_xcb(modifier_mask_t mask);

src/backend/x11/event.c

@@ -1,4 +1,4 @@
-#include "core/event.h"
+#include "common/event.h"
 
 #include <assert.h>
 #include <stdint.h>
@@ -12,9 +12,8 @@
 #include "backend/x11/common.h"
 #include "backend/x11/window.h"
 #include "base/memory.h"
-#include "core/backend.h"
-#include "core/types.h"
-#include "core/window.h"
+#include "interface/backend.h"
+#include "interface/event.h"
 #include "internal.h"
 
 static window_state_t *derive_window_states(

src/backend/x11/internal.h

@@ -7,8 +7,8 @@
 #include <xcb/xcb_keysyms.h>
 #include <xcb/xproto.h>
 
-#include "base/window_list.h"
-#include "core/event.h"
+#include "common/window.h"
+#include "interface/event.h"
 
 #define EWMH_ATOMS(X)                  \
   X(_NET_WM_NAME)                      \

src/backend/x11/window.c

@@ -12,8 +12,6 @@
 #include "backend/x11/cursor.h"
 #include "base/macros.h"
 #include "base/memory.h"
-#include "core/backend.h"
-#include "core/types.h"
 #include "internal.h"
 
 static char *window_get_text_property(

src/backend/x11/window.h

@@ -8,7 +8,7 @@
 #include <xcb/xproto.h>
 
 #include "base/app.h"
-#include "core/backend.h"
+#include "interface/backend.h"
 #include "internal.h"
 
 typedef struct atoms_t atoms_t;