Let systemd-oomd kill a runaway app instead of losing the session (#6445)

Nothing stood between "memory is tight" and "processes die at random":
the kernel OOM killer only fires after an allocation has already failed,
so a machine under pressure thrashes in reclaim until something breaks
somewhere fatal — seen in practice as Hyprland taking SIGBUS mid-memcpy
from a client shm pool that memory pressure was tearing down.

Enable systemd-oomd, which keys on PSI stall time rather than free
pages, and mark app.slice — and only app.slice — as its kill candidate.
Hyprland runs in session.slice, so the compositor is structurally
ineligible as a victim: oomd takes the app that caused the pressure and
the session survives. Thresholds (50% stall over 20s, Fedora's desktop
defaults) live in an oomd.conf.d drop-in; candidacy ships as a vendor
drop-in under /usr/lib/systemd/user so existing users get it on package
upgrade with no per-user seeding.

New installs enable the daemon from enable-services.sh; a migration
covers existing ones, restarting an already-running oomd so it doesn't
keep stale thresholds until reboot.

Deliberately not earlyoom: it triggers only when free RAM and free swap
are both under threshold, so Omarchy's large, mostly-idle swapfiles
keep it asleep through exactly this failure.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

docs/file-layout.md

@@ -105,6 +105,7 @@ default/**                     ──►  omarchy-settings    /usr/share/omarchy
   ├─ xdg-terminal-exec/*.list                           /usr/share/xdg-terminal-exec/
   ├─ applications/mimeapps.list                         /usr/share/applications/mimeapps.list
   ├─ systemd/user/*.{service,path}                      /usr/lib/systemd/user/
+  ├─ systemd/user/app.slice.d/10-oomd.conf              /usr/lib/systemd/user/app.slice.d/
   ├─ systemd/system-sleep/unmount-fuse                  /usr/lib/systemd/system-sleep/
   ├─ systemd/zram-generator.conf.d/90-omarchy.conf      /usr/lib/systemd/zram-generator.conf.d/
   ├─ fonts/omarchy/omarchy.ttf                          /usr/share/fonts/omarchy/