Remove old Theme struct and hand-curated color sections (litmus-i4kf)

Final cleanup once all consumers have migrated:

• Remove the old Theme struct from litmus-model (or rename ProviderColors to Theme if that’s cleaner)
• Remove old toml_format.rs parser (or repurpose for ThemeDefinition parsing)
• Delete hand-curated [colors] and [colors.ansi] sections from authored theme TOMLs
• Verify all tests pass, no references to old types remain

Depends on: web and CLI migrations complete

Plan

1. Update litmus-capture load_all_themes() to use load_themes_dir() instead of parse_toml_theme()
2. Remove unused parse_toml_theme import from capture crate
3. Verify no old-format theme TOMLs remain
4. Keep parse_toml_theme in litmus-model for CLI user-provided file support
5. Keep Theme struct as the runtime display model

Todo

• Update capture load_all_themes to use new format
• Remove unused imports
• Verify all theme files are in new format
• Zero warnings across all crates
• All tests pass

Summary of Changes

Updated litmus-capture’s load_all_themes() to use load_themes_dir() + ProviderColors::to_theme() instead of the old parse_toml_theme() path. Removed unused parse_toml_theme import from capture crate.

Kept in place:

• Theme struct (runtime display model, used everywhere)
• parse_toml_theme (still used by CLI for user-provided .toml files)
• defaults module (used by all format parsers)
• toml_format module (still needed)

All 60 theme definitions verified to be in new ThemeDefinition format.