For 'game-ey' math code, a matrix is at most 4x4 floats (64 bytes), that's fine for a value type that might live on the stack.

vec2..4 and matching matrix types up to 4x4 is basically also what's provided in GPU shading languages as primitive types, and personally I would prefer such a set of "SIMD-y" primitive types for Zig (maybe a bit more luxurious than @Vector, e.g. with things like component swizzling syntax - basically what this Clang extension offers: https://clang.llvm.org/docs/LanguageExtensions.html#vectors-...).