feat: add Parsl worker detection, casalog propagation, and parallel_map
- parslhelpers: add is_parsl_worker() (env var + argv detection), propagate casalog_file/log2term to tier0 workers via parsl_exec, improve ParslTask app naming, add parallel_map() for Parsl-aware fan-out with sequential fallback
- mpihelpers: defer task_args file deletion into _run() closure so Parsl worker crash/reschedule retry can re-read the file
- sessionutils: serialize context to disk when Parsl is ready, not only for MPI/Dask
- framework: is_top_level_task() returns False in Parsl workers
