Update litmus-web to render TermOutput instead of Scene (litmus-lm76)

Migrate litmus-web from Scene to TermOutput rendering:

• Embed output.{provider}.json files via include_str!() or include_bytes!()
• New TermOutputView component replaces SceneView:
  • Renders TermSpan as with inline CSS color
  • TermColor::Default → theme fg/bg from ProviderColors
  • TermColor::Ansi(n) → ProviderColors ANSI palette
  • TermColor::Indexed(n) → fixed 256-color → rgb CSS
  • TermColor::Rgb → literal rgb CSS
  • Bold/italic/dim/underline → CSS font-weight/style/opacity/decoration
• Provider selector switches which output.{provider}.json is rendered
• Update theme_detail.rs side-by-side to use TermOutputView on the left
• Update ScenePreview (theme list cards) to use TermOutput

Depends on: TermOutput types, fixture pipeline generating output files

Plan

1. Create crates/litmus-web/src/fixtures.rs — embed fixture output.json files, parse with LazyLock
2. Create crates/litmus-web/src/term_renderer.rs — TermOutputView and TermOutputPreview components
3. Wire TermOutputView into theme_detail.rs (replace SceneView on left panel)
4. Wire TermOutputPreview into theme_list.rs (replace ScenePreview in cards)
5. Wire TermOutputView into scene_across.rs and compare.rs
6. Tests for color resolution and fixture loading

Summary of Changes

• Created fixtures.rs module: embeds 8 fixture output.json files, OnceLock caching, fixture_by_id() and default_fixture() API (4 tests)
• Created term_renderer.rs module: TermOutputView and TermOutputPreview Dioxus components that render TermSpan with theme-aware inline CSS colors
• Updated theme_list.rs: TermOutputPreview for card previews (ScenePreview fallback)
• Updated theme_detail.rs: TermOutputView on left panel of side-by-side (SceneView fallback for neovim)
• Updated scene_across.rs: TermOutputView in grid cards
• Updated compare.rs: TermOutputView in comparison grid

Note: Contrast analysis tooltips are not available in TermOutput path (they were Scene-model-specific). Header stats still computed. Can be reimplemented as a follow-up.

Commits: 975c94d, c475a27, bc2393a