figma-transformer: decode Kiwi frame masks

[chubes4]

Summary

• selectively decode Kiwi NodeChange.frameMaskDisabled
• bridge frameMaskDisabled=false to the existing clips_content layout contract
• add synthetic contract coverage for selective decode and emitted clipping behavior

Testing

• composer test:contract

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: inspecting the FSE Kiwi payload, identifying a narrow missing field, implementing decode/normalization support, and adding contract coverage.