CultMathJson is the JSON twin of CultMathResolver: vectors, quaternion and
Color32 are objects keyed by field name, matrices are arrays of row objects,
rect is {min,max}. Consumers add them with AddCultMathConverters(); CultLib owns
no JSON options. CultCircle and CultSphere serialize and deserialize through
them, with [JsonConstructor] so the struct constructor is used. The Caching
README documents each MessagePack and JSON shape in one table.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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