Remove old Scene, ThemeColor, StyledSpan types and hand-written scenes (litmus-kbzo)

Final cleanup once all consumers migrated to TermOutput:

• Delete scene.rs (ThemeColor, StyledSpan, SceneLine, Scene)
• Delete scenes.rs (hand-written scene definitions)
• Remove scene_id_to_fixture_id() mapping (no longer needed)
• Clean up any remaining references to old types
• Verify all tests pass

Depends on: web and CLI migrations, contrast validation update

Plan

1. Add term_readability_score(theme, fixtures) to contrast.rs
2. Make fixtures::all_fixtures() pub in litmus-web
3. Update SceneMinimap to accept fixture metadata instead of Scene
4. Update sidebar to pass fixture data to minimap
5. Update all pages (theme_detail, scene_across, compare, theme_list) to iterate fixtures instead of scenes
6. Remove SceneView/ScenePreview fallback calls
7. Remove scene_renderer.rs module
8. Remove scene-based contrast functions and tests
9. Delete scene.rs and scenes.rs from litmus-model
10. Clean up all remaining references

Summary of Changes

Removed all Scene/ThemeColor/StyledSpan types and hand-written scene definitions:

• Deleted scene.rs and scenes.rs from litmus-model
• Deleted scene_renderer.rs from litmus-web
• Removed scene_id_to_fixture_id() mapping from screenshot_view.rs
• Removed ContrastIssue, validate_scene_contrast(), readability_score(), validate_theme_readability() and their tests from contrast.rs
• Updated scene_across.rs and compare.rs to iterate fixtures directly
• Updated theme_list.rs to use term_readability_score() and removed scene_renderer fallback
• Updated state.rs filter to use term_readability_score()
• Removed unused fixture_by_id() and its test
• All 177 tests pass, zero compiler warnings