feat(matlab): pad x-axis by half a day so points aren't glued to the edges

variation_plot: xlim([min(xd)-0.5, max(xd)+0.5]) on all learning-curve plots,
so the first point lifts off the y-axis (phase windows d6_*) and the last point
clears the right edge (d0_5 etc.). Regenerated all figures.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

analysis/matlab/variation_plot.m

@@ -45,6 +45,7 @@ set(gca, 'XTick', xd, 'XTickLabel', {}, 'Position', [0.2 0.30 0.60 0.60], ...
     'FontName', 'Arial', 'FontSize', 13, 'LineWidth', 1.5, 'Box', 'off', 'TickDir', 'out');
 xticklabels('auto')
 set(gca, 'XTickLabelRotation', 0);   % keep x labels horizontal (no auto-rotate)
+xlim([min(xd) - 0.5, max(xd) + 0.5]); % half-day padding so points aren't on the edges
 if strcmp(metric, 'count')       % success count: y-ticks every 5 units
     yl = ylim(gca);
     set(gca, 'YTick', floor(yl(1) / 5) * 5 : 5 : ceil(yl(2) / 5) * 5);