fix: implement automatic zorder assignment for proper trace layering

[huangzhibo]

Box Plot Layering Fix - Summary

Problem

Fix box plot layering issue where box plots always rendered behind other traces (ribbons, lines) despite correct layer ordering in ggplot2.

Solution

• Implement automatic zorder assignment based on ggplot2 layer order using new plotly.js functionality
• Add comprehensive test coverage for zorder functionality

Changes Made

1. Core Fix (R/ggplotly.R:1153-1162)

Added automatic zorder assignment loop that assigns sequential zorder values (zorder = i) to each trace based on their position in the data array.

2. plotly.js Update

Updated to v3.0.1 to support zorder functionality.

3. Test Coverage

Added test cases with visual regression tests to verify boxplot layering behavior.