Update litmus-cli to render TermOutput (litmus-0uoe)

Migrate litmus-cli mockup views from Scene to TermOutput:

• Load TermOutput data (bundled or from fixtures directory)
• Render TermSpan using crossterm colors:
  • TermColor::Default → crossterm reset
  • TermColor::Ansi(n) → crossterm AnsiValue
  • TermColor::Indexed(n) → crossterm AnsiValue
  • TermColor::Rgb → crossterm Rgb
• Minimal changes — CLI is simpler than web

Depends on: TermOutput types, fixture pipeline generating output files

Plan

1. Add resolve_with_theme() to TermColor in litmus-model (reuses existing indexed_color())
2. Add serde_json dep to litmus-cli
3. Write tests: resolve_with_theme unit tests, TermOutput-to-ratatui-Lines conversion test
4. Embed 3 fixture output.json files (git-diff, ls-color, shell-prompt) via include_str!()
5. Rewrite MockupsWidget to parse embedded fixtures, map TermSpan → ratatui Span using theme colors
6. Add term_color_to_ratatui() helper in util.rs

Summary of Changes

• Added TermColor::resolve_with_theme() to litmus-model for resolving terminal colors against a Theme (5 tests)
• Rewrote MockupsWidget to render real parsed TermOutput from embedded fixture JSON files
• Embedded 5 fixtures: git-diff, git-log, ls-color, cargo-build, shell-prompt
• Added Up/Down arrow keys to cycle between fixtures in the TUI
• Used LazyLock to cache parsed fixture data (avoids re-parsing on every frame)
• 8 new tests for color resolution, line conversion, modifier handling, and fixture loading

Commits: d3a0818, d5b902d, 64d5917, f45b930