Update contrast validation for TermColor (litmus-bcel)

Update contrast validation to work with TermOutput instead of Scene:

• TermColor::Default and Ansi(0-15) pairs: validate against theme palette (same as before)
• TermColor::Indexed/Rgb vs Default/Ansi: validate fixed color against theme-dependent counterpart
• TermColor::Indexed/Rgb vs Indexed/Rgb: skip (both fixed, theme-independent)
• New insight: can flag “this fixture uses hardcoded colors that clash with this theme’s background”
• Update ReadabilityIssue to reference TermSpan positions

Depends on: TermOutput types, web rendering migration

Plan

1. Add TermContrastIssue struct (references fixture_id and TermColor variants)
2. Add validate_term_output_contrast(output: &TermOutput, theme: &Theme) function
3. Skip pairs where both fg and bg are fixed (Indexed/Rgb vs Indexed/Rgb)
4. Validate all other pairs using APCA
5. Add validate_all_fixtures_contrast(fixtures: &[TermOutput], theme: &Theme) convenience
6. Tests: low-contrast detection, skip-fixed-pairs, dim exclusion

Summary of Changes

• Added TermContrastIssue struct with fixture_id and TermColor variant fields
• Added validate_term_output_contrast() — validates TermOutput spans using APCA
  • Skips fixed-color pairs (Indexed/Rgb vs Indexed/Rgb)
  • Skips Default/Default pairs (theme-controlled)
  • Skips dim and whitespace-only spans
• Added validate_fixtures_contrast() — aggregates across multiple fixtures
• 7 tests: ANSI detection, fixed-pair skip, fixed-on-theme, dim skip, default/default skip, Ansi-on-Ansi bg, multi-fixture aggregation

Commits: f868232, 53b2a5e