fix(plot): ensure correct per-facet axis bounds assignment in scatter plots (#140)

[sbuis]

Summary

Fix incorrect axis bounds assignment in plot() scatter panels caused by inconsistent
ordering between computed variable bounds and actual facet display order.

Problem

Axis limits were computed per variable using grouped data, but applied to facets
by positional index. When the ordering of variables in the data differed from
the facet display order in the ggplot object, axis limits were assigned to the
wrong panels.

This resulted in mismatched axis scaling across facets.

Root Cause

• compute_axis_bounds() returned unnamed vectors.
• make_axis_square() and force_y_axis() relied on positional matching.
• Facet order in ggplot depends on factor levels and plot build layout,
  which may differ from grouped data order.

Changes

• compute_axis_bounds() now returns named vectors indexed by variable name.
• Added internal helper get_facet_order() to retrieve actual facet layout
  order using ggplot_build().
• Updated make_axis_square() to reorder axis bounds according to
  the real facet display order.
• Updated force_y_axis() to apply the same logic.
• Removed implicit dependence on positional indexing.

Closes #140.

[github-actions]

📝 Styler has automatically formatted the following R files in this PR:

• R/specific_plotting_scatter.R

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