Marker trait for a symbolic matrix node whose children are its cells in row-major order and whose rebuild preserves the rows × cols shape.
Lets core and scalar algorithms distribute a scalar function element-wise over a symbolic matrix argument (exp(A), sin(A), … over a matrix with free-variable cells) without importing the matrix package — they see only this marker and the generic children / rebuild. Narrower than _ElementWise on purpose: only the matrix literal opts in. The dense counterpart (_MatrixValue) is handled numerically and is not marked.
