feat(doctor): #147 — flag machine state the flake never set; linger is the worked example

/var/lib survives a migration untouched, and a stray pre-Nomarchy
Linger=yes marker shaped the QA box's session behavior for six months
with nothing in the flake to show for it. Ship the class as guards:

- nomarchy-doctor: new machine-state-drift check after the flake-checkout
  check — loginctl show-user -p Linger; "yes" warns (nothing is broken —
  it is drift) and prints the disable-linger fix; unknown-to-logind skips.
- docs/MIGRATION.md § Post-migration cleanup: "/var/lib machine state
  survives migration" — the worked example and the general suspicion rule.
- checks.doctor: linger sub-case — warn fires under enable-linger, exit
  stays 0, warning clears after disable-linger.

Deliberately NOT hardening the session wiring against the portal-gtk
restart race linger enabled: a clean install cannot reach that state.
The Waybar exec-once loose thread survives as BACKLOG #149.

Verification: V2 — shellcheck clean, nix flake check --no-build green,
nix build .#checks.x86_64-linux.doctor green (both branches exercised).
Implementation by a Sonnet subagent; design and review on Fable.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

agent/BACKLOG.md

@@ -265,64 +265,16 @@ If that count is zero over a few weeks post-linger, **close this unbuilt**. If i
 is not, the upstream issue has sat ten months on vaxerski's "that will cause a
 segfault no?" and a reproducer would be worth more than a local guard.
 
-### 147. Nomarchy cannot see machine state it never set — and it bit the QA box for 6 months
+### 149. `[watch]` Waybar's exec-once workaround may be defending against nothing post-linger
 
-**Rewritten 2026-07-16 after diagnosis; the original filing was wrong and its
-wrongness is the point.** It claimed "a re-login leaves session units dead;
-the repo never adopted the standard session wiring". The repo *has* the wiring
-— HM's `wayland.windowManager.hyprland.systemd.enable` defaults on and is line
-1 of the generated `hyprland.conf` — and it fired correctly. Do not go add it.
-
-**What actually happened**, from systemd's own words at the 17:25:54 relogin:
-
-```
-Stopped target Hyprland compositor session.
-Requested transaction contradicts existing jobs: Transaction for
-graphical-session.target/stop is destructive (xdg-desktop-portal-gtk.service
-has 'start' job queued, but 'stop' is included in transaction).
-graphical-session.target: Failed to enqueue stop job, ignoring
-Reached target Hyprland compositor session.
-```
-
-`hyprland-session.target` stopped; `StopWhenUnneeded=yes` correctly tried to
-take `graphical-session.target` with it; systemd **refused the stop** because
-portal-gtk had a `start` job queued from its own post-crash restart loop (3
-restart/fail lines in that 13s gap). So graphical-session never went down, 8ms
-later hyprland-session came back to find it active, and nothing `WantedBy` it
-was re-pulled. cliphist had already burned 5 restarts into `start-limit-hit`;
-nm-applet/udiskie have no restart policy and just stayed dead.
-
-**But none of that can happen on a Nomarchy install.** The whole sequence needs
-the user manager to survive logout, i.e. `Linger=yes` — and linger came from an
-empty marker in `/var/lib/systemd/linger/bernardo` dated **2026-01-24**, which
-**pre-dates Bernardo's migration to Nomarchy** (he was on a custom flake then).
-Nothing here sets it: not modules, not hosts, not templates, and `git log -S`
-says it was never shipped, ever. Removed 2026-07-16 (`loginctl disable-linger`,
-no restart needed); nothing wanted it — `nomarchy-updates.timer` is
-`Persistent=yes` and catches up on login.
-
-**So the real item is the class, not the bug.** `/var/lib` is machine state; a
-migration reconciles the flake and leaves everything else exactly as it was.
-Nomarchy therefore ran for six months on its own primary V3 box with a setting
-it never chose, never set, and could not see — on the machine whose whole job is
-telling us how the distro behaves. That is a direct hit on the in-flake-state
-promise (state outside the checkout) *and* on V3's trustworthiness: a QA box
-that silently diverges reports on a distro nobody ships.
-
-**Pitch:** a `nomarchy-doctor` check for drift of this class — linger is the
-worked example (`loginctl show-user $USER -p Linger` vs. what the flake asks
-for), and MIGRATION.md should say plainly that `/var/lib` survives and what to
-sweep. Cost is small and it is the only part of this worth shipping. **Do not**
-harden the session wiring against the portal-gtk race: a clean install cannot
-reach it, and defending against it would be clever code for a state we do not
-ship.
-
-**Loose thread for whoever takes this:** `modules/home/hyprland.nix` runs Waybar
-from `exec-once` rather than a unit because the unit "raced Hyprland's IPC on a
-**warm** relogin ... never retried, so the bar vanished" — and a warm relogin is
-exactly what linger creates. That workaround may now be defending against
-nothing. Do not rip it out on this note alone; it works, and the claim needs a
-real relogin without linger to test.
+Spun out of #147 (shipped 2026-07-17) so its loose thread survives the entry:
+`modules/home/hyprland.nix` runs Waybar from `exec-once` rather than a systemd
+unit because the unit "raced Hyprland's IPC on a **warm** relogin ... never
+retried, so the bar vanished" — and a warm relogin is exactly what linger
+created. With the stray linger removed (2026-07-16) that workaround may be
+defending against nothing. Do not rip it out on this note alone: it works, and
+the claim needs a real relogin without linger to test (V3 — the same relogin
+that verifies #148's watcher fix would answer it).
 
 ### 120. A netinstall ISO, next to the fat offline one