fix: implement automatic zorder assignment for proper trace layering

This commit fixes the box plot layering issue where box plots always
rendered behind other traces (ribbons, lines) despite correct layer
ordering in ggplot2. Implements automatic zorder assignment based on ggplot2 layer order using new plotly.js functionality

Author: huangzhibo

R/ggplotly.R

@@ -1153,6 +1153,14 @@ gg2list <- function(p, width = NULL, height = NULL,
   l$cur_data <- ids[[layerData]]
   l$visdat <- setNames(lapply(return_dat, function(x) function(y) x), ids)
 
+  # automatically assign zorder values based on trace positionAdd commentMore actions
+  # this ensures proper layering (later traces on top) for ggplot2 without manual intervention
+  for (i in seq_along(l$data)) {
+    if (is.null(l$data[[i]]$zorder)) {
+      l$data[[i]]$zorder <- i
+    }
+  }
+
   l
 }