Make the idle-lock watcher survive a clean exit, and outlive a session restart

`swayidle-lock.service` was `enabled`, in `default.target.wants/`, and **dead for eight days**:

    Active: inactive (dead) since Mon 2026-10-12 16:22:23 PDT; 1 week 1 day ago
    Duration: 2d 21h 57min   Main PID: 3181 (code=exited, status=0/SUCCESS)
    Result=success  NRestarts=0

`Restart=on-failure` cannot see a clean exit, and `swayidle -w` exits 0 on SIGTERM. So the
watcher that owns both the 5-minute idle lock and the `before-sleep` lock stopped existing, and
nothing on the box noticed. The consequence is not cosmetic: with `HandleLidSwitch=suspend` and
`HandleLidSwitchExternalPower=""` (which falls back to it), **closing the lid suspended the
machine unlocked**, because the hook that locks on sleep lived inside the unit that had died. No
alternative was running: `pgrep -c -u blankpad 'swayidle|swaylock|hyprlock|gtklock'` = 0, and
neither `autostart.kdl` launches a locker.

Three settings, each with a job:

- `Restart=always` — a supervisor must not assume it can only die badly.
- `StartLimitIntervalSec=0` — with `always`, a fast loop must never wedge the unit into `failed`;
  a lock that keeps trying beats a lock that stopped trying.
- `PartOf=graphical-session.target` — it is wanted by `default.target`, so a session restart
  brought the target back without it; now the watcher is restarted with the session.

What triggered the 16:22 exit is **unattributed**: the user journal has no entries in that window,
so all that can be said is that swayidle exited with status 0 — precisely what the old policy was
blind to. The fix does not depend on knowing the trigger, which is the point.

Verification is behavioural, per the law recorded with it: start the unit, `kill -TERM` the main
PID, and watch `NRestarts` increment with a new `MainPID` — the October event, survived. Not
performed here: starting it arms a 5-minute idle lock on a machine that has gone a week without
one, and that is Frank's call. `systemd-analyze --user verify` clean, `bats tests/` 71/71,
`kb-lint.sh omp-docs` 0, `kb-coverage.py` 0.

Refs #47

dotfiles/systemd-user/.config/systemd/user/swayidle-lock.service

@@ -2,6 +2,13 @@
 Description=swayidle idle lock (locks via noctalia-shell IPC, suspends on lid)
 Documentation=https://github.com/swaywm/swayidle
 After=graphical-session.target
+# Restart together with the session, not merely at boot: this unit is wanted by
+# default.target, so a compositor restart used to leave it orphaned while the target came
+# back without it. Refs #47.
+PartOf=graphical-session.target
+# With Restart=always below, a fast failure loop must never wedge the watcher into `failed`:
+# a lock that keeps trying beats a lock that stopped trying. Refs #47.
+StartLimitIntervalSec=0
 
 [Service]
 # Locks via the noctalia v5 shell; `noctalia msg` resolves the running instance
@@ -11,7 +18,10 @@ ExecStart=/usr/bin/swayidle -w \
   timeout 300 '/usr/bin/noctalia msg session lock' \
   before-sleep '/usr/bin/noctalia msg session lock' \
   after-resume '/usr/bin/noctalia msg session lock'
-Restart=on-failure
+# `swayidle -w` exits 0 on SIGTERM, and that is how this watcher died on 2026-10-12: a clean
+# exit that `Restart=on-failure` cannot see, leaving the box unlocked for a week. A supervisor
+# restarts always. Refs #47.
+Restart=always
 RestartSec=5s
 
 [Install]