v0.15.0 — GAM smoothing, stats→anofox migration, ggpubr parity

GAM smoothing

• geom_smooth(method = "gam") — penalized B-spline (P-spline) smoother via anofox-regression (GCV-selected λ, Bayesian t-CI). SmoothMethod::Gam / GeomSmooth::gam().
• Hover tooltips on the fitted line (ŷ + CI) in the native SVG backend.

Statistics migrated to the anofox crates (this crate stays stats-free)

• qt t-quantile → anofox StudentsT; qnorm → anofox Normal; confidence-ellipse radius → ggpubr's F-based formula via FisherSnedecor. Non-regression builds keep documented fallbacks.

ggpubr parity (new ggpubr feature → anofox-statistics)

• ggsci journal palettes (npg/aaas/nejm/lancet/jama/jco/d3) + theme_pubr().
• stat_cor() (Pearson/Spearman R + p) and stat_compare_means() (Wilcoxon/Kruskal auto; +t/ANOVA), via a new Stat::panelwise() capability.
• geom_bracket() and auto-pairwise brackets: stat_compare_means_pairwise(comparisons).
• One-call constructors: ggscatter/ggline/ggboxplot/ggviolin/gghistogram/ggdensity.
• ggarrange() multi-plot composition — SVG and native PNG.

Other

• New polars-free examples/ggpubr_gallery.rs + gallery images and README "Publication-ready" section.
• CI covers the ggpubr feature.
• Fix: bump ethnum → 1.5.3 (compile on current stable rustc).

Full history: #110