test(archsetup): port full shell validation sweep to Testinfra (P2)

Port all ~26 post-install checks from validation.sh to pytest/Testinfra,
reaching parity before the cutover. Adds test_users, test_packages,
test_services, test_desktop, test_boot, test_keyring, and test_archsetup
(88 tests after parametrizing groups, services, timers, tools, and configs),
plus shared conftest fixtures for ZFS/NVMe/compositor/networking gating.

The shell sweep's three outcomes map cleanly: hard failures become assertions,
advisory warnings and unmet preconditions (headless compositor, slirp
networking, optional services, non-ZFS/non-NVMe hosts) become skips.

One correctness fix vs the shell sweep: check awww, not swww — archsetup
installs awww (swww's successor) and `pacman -Q swww` no longer matches.

Verified on the host: py_compile clean, pytest --collect-only green (88 tests).
The sweep against a real VM is verified by the make test run that follows.

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