Add ThemeDefinition and ProviderColors types to litmus-model (litmus-jmna)

Add new types to litmus-model alongside the existing Theme struct (don’t remove it yet):

• ThemeDefinition: name, variant (dark/light), slug, providers (HashMap<String, String>)
• ProviderColors: provider slug, source_version, all 22 color fields (same as current Theme colors)
• TOML deserialization for both: .toml for definitions, .{provider}.toml for provider colors
• Loader function that scans a themes directory and returns Vec<ThemeDefinition> + HashMap<(slug, provider), ProviderColors>

Keep the old Theme struct and parsers intact — they’ll be removed in a later task after consumers migrate.

Plan

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

1. Variant enum (Dark/Light) with serde lowercase
2. ThemeDefinition struct: name, variant, slug (derived from filename), providers HashMap
3. ProviderColors struct: provider slug, source_version, same color fields as Theme
4. TOML parsing for both types (reuse existing parse_field pattern)
5. load_themes_dir() function: scan directory, return Vec + HashMap<(String, String), ProviderColors>

Test-first approach

• Write tests for Variant serde, ThemeDefinition parsing, ProviderColors parsing, and loader function
• Then implement to make them pass

Commits

1. Tests commit (failing)
2. Implementation commit

Summary of Changes

Added provider module to litmus-model with:

• Variant enum (Dark/Light) with lowercase serde
• ThemeDefinition struct: name, variant, slug (from filename), providers HashMap<String, String>
• ProviderColors struct: provider slug, source_version, all 21 color fields matching Theme
• TOML parsing for both types via parse_theme_definition() and parse_provider_colors()
• load_themes_dir() recursive directory loader returning Vec + HashMap<(slug, provider), ProviderColors>
• Shared parse_hex_color() helper extracted to lib.rs, reused across toml_format and provider modules
• Filename/TOML provider validation in the loader
• 20 tests covering parsing, serde, edge cases, directory loading, error propagation

Old Theme struct left intact as specified.