[External] Synty Polygon 외부 에셋 패키지 추가

Ultraworked with [Sisyphus](https://github.com/code-yeonggu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

Assets/External/PolygonGeneric/Shaders/Generic_ParticlesUnlit.shadergraph.meta

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