A Base1Solid (or OrigTextureId==0) Surface can carry a null ColorValue;
DecodeSolidColor dereferenced it (color.Alpha) and threw NullReferenceException.
It is called directly from TextureCache.DecodeFromDats, OUTSIDE
DecodeRenderSurface's try/catch, so the NRE crashed the whole client. Surfaced
by the D.2b chrome prove-out feeding UI surface ids. Guard null -> Magenta
(the decoder's existing "undecodable" sentinel). Test added.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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