v0.2.0

• Added Layout field expansion — Phase 1 & 2
  • Phase 1 (Basic fields):
    • Spacing: bar_gap, bar_group_gap, box_gap, box_group_gap
    • Interaction: hover_mode, drag_mode, click_mode
    • Sub-objects: font (family/size/color), coloraxis (cmin/cmax/cmid/auto/reverse/show)
  • Phase 2 (Axis support):
    • Default axes: xaxis, yaxis with full field support
    • Named axes: xaxis2–xaxis8, yaxis2–yaxis8
    • Axis fields: title, show_grid, show_line, zero_line, visible,
      anchor, overlaying, range, color, line_color, grid_color,
      tick_prefix, tick_suffix, tick_format, hover_format, category_array,
      fixed_range, scale_anchor, auto_margin, show_tick_labels
    • Axis enums: category_order (16 variants), type (linear/log/date/category/multicategory)
• Added helper functions: parse_axis_obj(), parse_named_axes()
• Added 3 axis tests (xaxis, date type, named axes)
• Updated documentation (USAGE-zh_CN.md, USAGE.md) with new Layout fields and Axis reference