feat(window): 无标题窗口标题按优先级回退显示 instance/class/app_id

新增 window_get_showing_title，按 title → instance_name →
class_name → app_id 优先级取显示标题，窗口快照改用该函数，
避免标题缺失时显示为空。

src/core/listeners.c

@@ -5,6 +5,7 @@
 #include "core/binding.h"
 #include "core/layout.h"
 #include "core/state.h"
+#include "core/window.h"
 
 #define FOR_EACH_LISTENER(FIELD, BODY)         \
   do {                                         \
@@ -117,7 +118,7 @@ static bool listeners_window_snapshot(
   *out = (zdwm_window_t){
     .workspace = window->workspace_id,
     .id = window->id,
-    .title = window->title,
+    .title = window_get_showing_title(window),
     .focused = workspace->focused_window_id == window->id,
     .urgent = window->urgent,
     .skip_taskbar = window->skip_taskbar,

src/core/window.c

@@ -145,3 +145,11 @@ bool window_can_resize(const window_t *window) {
 
   return true;
 }
+
+const char *window_get_showing_title(const window_t *window) {
+  if (window->title) return window->title;
+  if (window->instance_name) return window->instance_name;
+  if (window->class_name) return window->class_name;
+  if (window->app_id) return window->app_id;
+  return nullptr;
+}

src/core/window.h

@@ -67,3 +67,4 @@ bool window_need_resize(const window_t *window, int32_t width, int32_t height);
 bool window_should_has_border(const window_t *window);
 bool window_should_fix_size(const window_t *window);
 bool window_can_resize(const window_t *window);
+const char *window_get_showing_title(const window_t *window);