feat: add sticky error-aware key mastery

Persist ranked-only mastery and use it consistently across progression, milestones, counts, and localized UI. Preserve mastery during history rebuilds, replay errors accurately, and generate replay-derived test profiles. Document the follow-up plan to remove the drill history cap.

src/ui/components/stats_sidebar.rs

@@ -4,9 +4,9 @@ use ratatui::style::Style;
 use ratatui::text::{Line, Span};
 use ratatui::widgets::{Block, Paragraph, Widget};
 
+use crate::i18n::t;
 use crate::session::drill::DrillState;
 use crate::session::result::DrillResult;
-use crate::i18n::t;
 use crate::ui::theme::Theme;
 
 pub struct StatsSidebar<'a> {
@@ -198,7 +198,10 @@ impl Widget for StatsSidebar<'_> {
 
             if prior_count > 0 {
                 lines.push(Line::from(vec![
-                    Span::styled(vs_avg_label.as_ref(), Style::default().fg(colors.text_pending())),
+                    Span::styled(
+                        vs_avg_label.as_ref(),
+                        Style::default().fg(colors.text_pending()),
+                    ),
                     Span::styled(wpm_delta_str, Style::default().fg(wpm_delta_color)),
                 ]));
             }