0006: Supervise the system sort for the remaining sort routes

Status: accepted.

Fastmash sorts in process, with disk spill, for the common operations and locales. For the remaining combinations it uses the system sort, as GNU datamash does, but never directly: a small supervisor process starts sort with a fixed argument list in a private temporary directory, watches the main process through a Linux pidfd, and cleans up even if the main process is killed.

Consequences: Fastmash ships two executables. The external route is used only where it works: Linux 5.11 or later, a GNU or uutils coreutils sort, the supervisor installed beside fastmash, and the other conditions in System requirements. Elsewhere those jobs sort in process, with the same output. The in-process sorter is expected to take over the remaining routes over time.