boost::container::map::operator[] default-constructs the mapped_type
when the key doesn't exist. TaskRecord lacked a default constructor
after the e21b2af refactor (only had an allocator-arg constructor).
Added a static vec_allocator_f (matching RuleStatShm.h pattern) and a
default constructor that initializes data_record with it.
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.