feat(aur): list and upgrade AUR packages through paru or yay on Arch

On the pacman backend, an installed AUR helper (paru, else yay;
--aur-helper=paru|yay|none, --no-aur) adds the AUR updates to the review
list. The helper never runs as root: when the tool is root through sudo it
runs as SUDO_USER (runuser -u, else sudo -u, with that user's HOME, from
that home); unprivileged it runs as is; a root login gets no AUR, with a
warning. The banner names the helper ("+ AUR via paru").

- listing: <helper> -Qua (also with --no-refresh), rows with repository
  "aur"; [ignored] is shown [held]; a failing -Qua warns and the session
  goes on with the repository rows
- every row has a marker column after the number: "*" for AUR rows, a
  space otherwise (on every backend)
- N?? merges pacman -Qi with <helper> -Sia; N?, N+, Nx stay with pacman;
  N- excludes through the helper's --ignore
- <enter>: the repository step exactly as before, then <helper> -Sua
  --ignore <excluded> as the invoking user (--noconfirm only when stdin is
  not a terminal); pacman is skipped when only AUR rows are left; a helper
  failure is reported and its status returned; --dry-run prints it
- other backends never run a helper; --aur-helper is ignored with a warning

paru 2.1.0 and yay 13.0.1 formats verified in discovery/pacman.md section
15 (proof: discovery/pacman/aur.sh). Unit tests replay the recorded output
(fake paru, yay, runuser, getent); the archlinux E2E run installs the
foreign su-dummy-aur and drives a fake paru through a fake sudo built on
passwordless su for wheel.

tests/unit/data/aur/Qua-fail-yay.err

@@ -0,0 +1,4 @@
+ -> 1 error occurred:
+	* request failed: Get "https://aur.archlinux.org/rpc?arg%5B%5D=paru&arg%5B%5D=pfetch&arg%5B%5D=su-dummy-aur&type=info&v=5": dial tcp 127.0.0.1:443: connect: connection refused
+
+