Compose two operators.
Details
When both operands are built-in explicit matrices (dense double
or Matrix storage), the product is formed once and wrapped as a native
explicit operator (metadata$fused == "compose") only when that is cheap
and memory-safe: always for a small product (at most 65,536 entries);
for a larger dense product only when it holds no more entries than the
two factors together and costs at most 2^30 multiply-adds; for a larger
sparse product only when a structural bound on its nonzeros stays within
four times the factors' nonzeros and below a quarter of its entries, so
a sparse product is never densified. Otherwise (and for callback or
mixed operands) the result is a lazy composition that applies B then
A and stores no product.