Returning end-of-input from chunks::read on a disconnected receiver
also masked a panicking ext_sort sorter thread (comparator or Rayon
panic in sort_by), letting sort exit 0 with truncated or empty output.
Retain the sorter JoinHandle and join it after read_write_loop, mapping
a thread panic to an error; drop the sorted-chunk receiver first so a
still-running sorter unblocks instead of deadlocking the join. Added an
external multi-chunk sort test covering the spill-to-files join path.
Fixes#6736
roboomp
b351c8e467