extend pool classes to 16 MiB for whole-body assembly buffers
Body consumers collect a stream into one final allocation the size of
the entire body; at 8-16 MiB those fell through the pool to
alloc_zeroed and paid page faults during assembly. Pool them like the
streaming chunks (classes at least CLASS_CAP_BYTES big retain a single
block, bounding worst-case retention at 96 MiB).
