Fix JSON encoding of Figma archive reports

[chubes4]

Summary

• omit raw archive asset bytes from report metadata while preserving generated export bytes
• omit invalid UTF-8 decoded archive metadata rather than applying lossy substitution
• cover a synthetic binary archive transform and worker-style JSON encoding

Testing

• composer test (from figma-transformer)
• php -l figma-transformer/src/FigmaTransformer.php
• php -l figma-transformer/tests/contract/KiwiParserContract.php

Fixes #1654

AI assistance: GPT-6 Astra was orchestrated with OpenCode to investigate, implement, and test this repair; human direction and review guide the work.