fix(matlab): adopt user's compact plot layout for all variations

Port the unmerge_d0_5 fix (b6dc69c) into variation_plot.m: tight centred plot
box (Position [0.2 0.30 0.60 0.60]), TickDir out, and xticklabels('auto') --
which keeps x labels horizontal when they fit (<=~10 days) and auto-rotates
them when crowded (e.g. 14-day windows). Replaces the manual rotated-text hack.
Regenerated all learning-curve figures; removed the stray plotcurve.asv and
gitignored *.asv.

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

.gitignore

@@ -29,3 +29,4 @@ playwright-report/
 # OS
 .DS_Store
 Thumbs.db
+*.asv