Add readability badges and contrast markers to compare page (litmus-962t)


Status	Done · `archived`
Type	Task
Priority	normal
Parent	UI/UX improvement: shortlist, side-by-side, and contrast issues (litmus-ysy5)

Wire contrast issues into the compare page — the highest-value change.

Call validate_fixtures_contrast() per theme in compare.rs
Add ScoreRing (readability %) to each column header
Add issue count badge next to readability score
Pass issue_details to TermOutputView (enables span markers, tooltips, footnotes)
Add per-fixture issue count badge on fixture headers in compare grid

Summary of Changes

Added ThemeContrastData struct and compute_theme_contrast() helper
Column headers now show ScoreRing readability % and issue count badge
TermOutputView receives issue_details, enabling span markers, tooltips, and footnotes
Per-fixture headers show worst-case issue count across compared themes
Review fix: aligned issue count calculation with detail page (rules.len())