Contrast issues feature improvements (litmus-jzjb)


Status	Done · `archived`
Type	Feature
Priority	normal

Add stable slugs, scene tab badges, visual-first issue display, and scene span markers

Summary of Changes

1. Model: Stable slugs for issue types

Added ThemeColor::slug() method returning short identifiers (“fg”, “bg”, “ansi1”, etc.)
Added slug, fg_color, and bg_color fields to ContrastIssue
Slug format: "scene-id/fg-slug-on-bg-slug" (e.g. "cargo-build/ansi1-on-bg")
Populated from span fg/bg in validate_scene_contrast

2. Scene tab badges

Built HashMap<&str, usize> counting issues per scene
Scene tab buttons show a pill badge with count when > 0
CSS: small red pill using --app-error background

3. Visual-first issue display

Deduplicated issues by slug within each scene group
Replaced text-heavy layout with colored sample spans showing actual fg/bg
Ratio shown prominently, hex codes dimmer as secondary info
Scene group headers are clickable buttons that navigate to that scene tab

4. Scene span markers

Added issue_spans: Vec<(usize, usize)> prop to SceneView
Threaded through LineView (line_idx) → SpanView (has_issue)
CSS class contrast-issue-span with dashed red outline
theme_detail.rs filters issues to current scene and builds the (line, span) vec
ScenePreview and AllScenesView use defaults (no markers)