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.
The workaround was needed because bipc::string items in shared memory would
segfault on restart when tag names exceeded SSO length. Now that display
data (items, etc.) lives in local-memory DisplayCache and only cold doubles
remain in shared memory, the dangling-allocator bug no longer exists.
Deleting the file also broke mon-cron IPC across restarts.
