Persistent aria2c as a systemd user service (pos-aria2.service, enable --now,
linger warning) on localhost:6800 with a generated RPC secret in
~/.config/linux_post_install/download.env (chmod 600, env override). 18
commands: start/stop/status, add/torrent/metalink (auto-start, --tmux live
view), list/info/files/peers, pause/resume/remove/purge/move, limit/set,
watch (2s live repoll). JSON built via jq -nc --arg, never string
interpolation. Deps: aria2 in preinstall PACKAGES, aria2c/jq/curl guards
before --help. Docs: POS.md + howto/network.md + indices + Common Tasks row.
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