Fix memory leaks in widget/window lifecycle

This adds proper destroy event handling throughout the widget hierarchy
to fix memory leaks:
- box: Propagate destroy events to all child widgets before destruction
- widget: Add TwinEventDestroy case to base widget dispatch
- label: Free dynamically allocated label text on destroy
- window: Invoke destroy callbacks to allow proper resource cleanup
- pixmap: Initialize window field to prevent undefined behavior

Author: jserv

src/box.c

@@ -158,6 +158,17 @@ twin_dispatch_result_t _twin_box_dispatch(twin_widget_t *widget,
         _twin_widget_dispatch(widget, event) == TwinDispatchDone)
         return TwinDispatchDone;
     switch (event->kind) {
+    case TwinEventDestroy:
+        /* Destroy all children first */
+        while (box->children) {
+            child = box->children;
+            box->children = child->next;
+
+            /* Send destroy event to child */
+            ev.kind = TwinEventDestroy;
+            (*child->dispatch)(child, &ev);
+        }
+        break;
     case TwinEventQueryGeometry:
         return _twin_box_query_geometry(box);
     case TwinEventConfigure:

src/label.c

@@ -73,6 +73,11 @@ twin_dispatch_result_t _twin_label_dispatch(twin_widget_t *widget,
     case TwinEventQueryGeometry:
         _twin_label_query_geometry(label);
         break;
+    case TwinEventDestroy:
+        /* Free the label text */
+        if (label->label)
+            free(label->label);
+        break;
     default:
         break;
     }

src/pixmap.c

@@ -51,6 +51,7 @@ twin_pixmap_t *twin_pixmap_create(twin_format_t format,
 #if defined(CONFIG_DROP_SHADOW)
     pixmap->shadow = false;
 #endif
+    pixmap->window = NULL; /* Initialize window field */
     pixmap->p.v = pixmap + 1;
     memset(pixmap->p.v, '\0', space);
     return pixmap;
@@ -80,6 +81,7 @@ twin_pixmap_t *twin_pixmap_create_const(twin_format_t format,
     pixmap->origin_x = pixmap->origin_y = 0;
     pixmap->stride = stride;
     pixmap->disable = 0;
+    pixmap->window = NULL; /* Initialize window field */
     pixmap->p = pixels;
     return pixmap;
 }

src/widget.c

@@ -96,6 +96,9 @@ twin_dispatch_result_t _twin_widget_dispatch(twin_widget_t *widget,
         _twin_widget_paint(widget);
         widget->paint = false;
         break;
+    case TwinEventDestroy:
+        /* Base widget has no special cleanup */
+        break;
     default:
         break;
     }

src/window.c

@@ -95,6 +95,11 @@ twin_window_t *twin_window_create(twin_screen_t *screen,
 void twin_window_destroy(twin_window_t *window)
 {
     twin_window_hide(window);
+
+    /* Call the destroy callback if set to clean up window contents */
+    if (window->destroy)
+        (*window->destroy)(window);
+
     twin_pixmap_destroy(window->pixmap);
     free(window->name);
     free(window);