fix: support nested token declarations in frontmatter

[vikks]

This PR fixes a bug where nested color declarations in the DESIGN.md frontmatter (e.g., background: { light: '#fff', dark: '#000' }) caused validation errors.

Key Changes

• Recursive Parsing: Implemented a forEachLeaf helper in the ModelHandler to handle arbitrary token nesting depths.
• Flattened Symbol Table: Nested tokens are now correctly flattened (e.g., colors.background.light) for resolution and validation.
• Improved Type Safety: Updated ParsedDesignSystem to support nested objects in tokens.
• Comprehensive Tests: Added unit tests for 2, 3, and 4-level nesting, as well as mixed nested/flat declarations.

Fixes #102\n\n### Update (June 5, 2026)\n- Fixed TypeScript error by expanding ResolvedValue to include number and boolean.\n- Added a token nesting depth limit of 20 levels (MAX_TOKEN_NESTING_DEPTH) to ensure performance and prevent excessive recursion.

• Made these limits configurable via spec-config.yaml (limits section).

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