Add TermColor, TermSpan, TermLine, TermOutput types to litmus-model (litmus-q9lp)

Add the new terminal output types to litmus-model:

• TermColor enum: Default, Ansi(u8), Indexed(u8), Rgb(u8, u8, u8)
• TermSpan: text + fg/bg TermColor + bold/italic/dim/underline
• TermLine: Vec
• TermOutput: id, name, cols, rows, Vec
• Serde JSON serialization/deserialization for all types
• TermColor resolution method: resolve(provider_colors) → CSS-ready rgb values
• Indexed(16-255) → fixed RGB lookup table (standard 256-color palette)

Keep existing Scene/ThemeColor types — removed in a later task.

Plan

New file: crates/litmus-model/src/term_output.rs

1. TermColor enum: Default, Ansi(u8), Indexed(u8), Rgb(u8, u8, u8)
2. TermColor::resolve() method: takes ProviderColors, returns Color
   • Default → uses context (caller decides fg vs bg)
   • Ansi(0-15) → ProviderColors.ansi lookup
   • Indexed(16-255) → fixed 256-color palette lookup table
   • Rgb → direct Color
3. TermSpan: text + fg/bg TermColor + bold/italic/dim/underline
4. TermLine: Vec
5. TermOutput: id, name, cols, rows, Vec
6. Serde JSON serialization/deserialization for all types

256-color palette

Standard xterm-256 color palette: colors 16-231 are a 6x6x6 color cube, 232-255 are grayscale.

Commits

1. Tests + implementation
2. Review fixes

Summary of Changes

Added term_output module to litmus-model with:

• TermColor enum: Default, Ansi(0-15), Indexed(16-255), Rgb(r,g,b) with tagged serde
• TermColor::resolve(): maps to concrete Color via ProviderColors + default fallback
• Standard xterm-256 color palette: 6x6x6 cube (16-231) + grayscale ramp (232-255)
• TermSpan: text + fg/bg TermColor + bold/italic/dim/underline with skip_serializing_if
• TermLine and TermOutput: structured parsed terminal output
• 11 tests covering resolution, palette corners, and JSON serde round-trips

Existing Scene/ThemeColor types kept intact.