feat(bar): bar item 扩展钩子功能 bar cell 添加 fixed_width 属性与操作接口

- item type 加入可为空的 after_layout 和 after_draw 钩子
- cell 加 fixed_width 属性， cell api 添加 cell_set_fixed_width 和
  cell_get_region 两个接口
- cell api 改为 const 避免外部修改

include/zdwm/bar.h

@@ -48,6 +48,11 @@ typedef struct zdwm_bar_config_t {
 
 typedef struct zdwm_bar_item_t zdwm_bar_item_t;
 
+typedef struct zdwm_bar_x_region_t {
+  int32_t start;
+  int32_t end;
+} zdwm_bar_x_region_t;
+
 typedef struct zdwm_bar_cell_api_t {
   size_t (*get_cell_count)(zdwm_bar_item_t *item);
   void (*set_cell_count)(zdwm_bar_item_t *item, size_t count);
@@ -57,6 +62,13 @@ typedef struct zdwm_bar_cell_api_t {
   void (*cell_set_fg)(zdwm_bar_item_t *item, size_t index, const char *color);
   void (*cell_set_icon)(zdwm_bar_item_t *item, size_t index, zdwm_icon_t icon);
   void (*cell_set_indicator)(zdwm_bar_item_t *item, size_t index, bool show);
+  void (*cell_set_fixed_width)(
+    zdwm_bar_item_t *item,
+    size_t index,
+    int32_t width
+  );
+
+  zdwm_bar_x_region_t (*cell_get_region)(zdwm_bar_item_t *item, size_t index);
 } zdwm_bar_cell_api_t;
 
 typedef struct zdwm_bar_click_params_t {
@@ -68,6 +80,13 @@ typedef struct zdwm_bar_click_params_t {
   void *state;
 } zdwm_bar_click_params_t;
 
+typedef struct zdwm_bar_item_hook_params_t {
+  zdwm_bar_item_t *item;
+  const zdwm_bar_cell_api_t *cell_api;
+  zdwm_bar_x_region_t region;
+  void *state;
+} zdwm_bar_item_hook_params_t;
+
 typedef struct zdwm_bar_item_type_t {
   void *(*create_state)(zdwm_output_id_t output_id, void *config);
   void (*update)(
@@ -75,6 +94,8 @@ typedef struct zdwm_bar_item_type_t {
     const zdwm_bar_cell_api_t *cells,
     void *state
   );
+  void (*after_layout)(const zdwm_bar_item_hook_params_t *params);
+  void (*after_draw)(const zdwm_bar_item_hook_params_t *params);
   zdwm_action_t (*on_click)(zdwm_bar_click_params_t *params);
   void (*destroy_state)(void *state);
   zdwm_bar_item_t *instance;