feat(core): Abstract design data sources behind per-asset-type protocols

[alexey1312]

Description

• Introduce ColorsSource, ComponentsSource, and TypographySource protocols in ExFigCore to decouple the export pipeline from Figma-specific loaders
• Add DesignSourceKind enum (.figma, .penpot, .tokensFile, .tokensStudio, .sketchFile) for explicit source dispatch
• Refactor ColorsSourceInput to use sourceKind + sourceConfig: any ColorsSourceConfig instead of flat Figma/tokens-file fields
• Add sourceKind field (default .figma) to IconsSourceInput, ImagesSourceInput, TypographySourceInput
• Extract FigmaColorsSource, TokensFileColorsSource, FigmaComponentsSource, FigmaTypographySource from context implementations into dedicated Source/ directory
• Add SourceFactory for centralized source creation by DesignSourceKind
• Refactor all 4 context implementations to accept injected source protocols via constructor
• Update all Plugin*Export files to create sources via factory
• Add SourceKind typealias and sourceKind field to PKL schemas (FrameSource, VariablesSource)
• Add sourceKind resolution priority: explicit PKL value > auto-detect (tokensFile presence) > default .figma
• Update entry bridge methods in all 4 platform modules to pass sourceKind

Additional notes

• Download commands (DownloadColors, DownloadIcons, DownloadImages) and MCP exfig_download handler are deferred — they use separate loader code paths not covered by this refactoring
• Granular cache path in IconsExportContextImpl/ImagesExportContextImpl retains direct client usage alongside injected ComponentsSource
• Full backward compatibility: all existing PKL configs work without changes, sourceKind defaults to .figma
• 1828 tests pass, lint clean, format clean

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a significant refactoring to abstract design data sources behind protocols, enhancing flexibility and maintainability. It decouples the export pipeline from Figma-specific loaders by introducing ColorsSource, ComponentsSource, and TypographySource protocols. A SourceFactory is added for centralized source creation, and existing context implementations are refactored to accept injected source protocols. This change maintains backward compatibility and ensures existing PKL configurations work without modification.

Highlights

• Design Source Abstraction: Introduced protocols for different design sources (Figma, Tokens File, etc.) to decouple the export pipeline from specific loaders.
• SourceFactory: Implemented a centralized factory for creating source instances based on DesignSourceKind.
• Context Refactoring: Refactored export contexts to accept injected source protocols, promoting loose coupling and testability.
• PKL Schema Updates: Added sourceKind to PKL schemas for FrameSource and VariablesSource, enabling explicit source dispatch.

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[gemini-code-assist]

Code Review

This pull request introduces a significant and valuable abstraction layer for data sources, decoupling the export pipeline from Figma-specific loaders. The introduction of *Source protocols, a SourceFactory, and updates to the PKL schema are well-executed. However, there are some critical issues in the implementation of the plugin export subcommands where the new sources are instantiated. The new SourceFactory is not used, and Figma...Source implementations are hardcoded, which defeats the purpose of the abstraction. Additionally, a useful warning for users has been lost in the refactoring. My review includes detailed feedback on these points.