feat: pos network download restart + smart retry + systemd healer — outage-resilient downloads

restart <gid>: re-queue from history — torrents via rebuilt magnet
(urn:btih: + &tr= trackers), HTTP via original URIs keeping dir/out;
--continue=true resumes partials, complete files verify instantly.

retry <gid|all>: waits out internet outages (NET_PROBE seam,
--interval/--max-wait), re-queues and re-verifies; aria2 error 3 = real
problem → diagnosed + marked permanent (url:/bt: ids in download.retry,
skipped by retry all, manual restart overrides); --once/--quiet for the
healer timer.

Healer: pos-aria2-retry.{service,timer} user units — arms on download
start (add/torrent/metalink/restart), disables when nothing left.
watch <gid> auto-restarts after an outage.

Fixes from stub-suite review: ensure_healer missing from submit paths;
RESTART_NAME lost across do_restart subshell (download_name helper);
restart exited 1 (tmux test as last statement).

Stub harness (/tmp/opencode/dl-test) 119/119 green; make gen && make check green.
Docs: POS.md rows, howto/network.md outage recipe, SYSTEMD.md user units.

DOC/AGENT_Context_Project.md

@@ -591,7 +591,7 @@ Use conventional prefixes: `feat:`, `fix:`, `docs:`, `refactor:`, `chore:`
 | `bin/pos-media-mp3` | 80 | Download audio as MP3 (yt-dlp) |
 | `bin/pos-media-mp4` | 126 | Download video as MP4 (smart/interactive format select) |
 | `bin/pos-network-checkport` | 496 | Check TCP/UDP port reachability (nmap, or bash/nc fallback) + local interface view |
-| `bin/pos-network-download` | 558 | aria2 RPC daemon + queue control (add/torrent/metalink, watch, limits) |
+| `bin/pos-network-download` | 874 | aria2 RPC daemon + queue control (add/torrent/metalink, watch, limits) |
 | `bin/pos-network-hotspot` | 93 | Wi-Fi hotspot via create_ap + wihotspot-gui |
 | `bin/pos-network-ip` | 69 | Show interfaces, routes, public IP + location |
 | `bin/pos-network-scan` | 271 | Parallel ping sweep of CIDR |