Commit e21b2af changed TaskShm map value from DataRecord (flat array)
to TaskRecord (struct wrapping shm_vector_f), but three call sites in
exp_base.cpp didn't drill into the .data_record member — they called
size()/operator[]/push_back() on TaskRecord itself, which has none.
DataRecord used a fixed float[129600000] consuming 5GB disk even when
collecting only a few hundred data points. Replaced with shm_vector_f
that grows on demand via push_back. Removes the need for rm -rf on
process exit — vector destructor frees memory back to the segment.
Also drops now-unnecessary task_data_size member.
