feat(stability): default-on memory-pressure protection (earlyoom)

modules/nixos/oom.nix — running out of memory now kills the offending
process (with a desktop notification via systembus-notify) instead of
freezing the desktop for minutes until the kernel OOM killer fires.

earlyoom over systemd-oomd, deliberately: oomd kills whole cgroups,
and a Hyprland session runs as ONE scope (nothing here spawns per-app
systemd scopes, unlike GNOME) — under pressure oomd would take out the
entire desktop to save it. earlyoom kills the single largest process
before the thrash point. nixpkgs default-enables oomd in an inert
state (no slices monitored); it is disabled outright so there is one
owner. Session plumbing is --avoid-listed (unanchored — NixOS wrappers
rename comm to .foo-wrapped); no --prefer tuning, largest-RSS
selection already finds the hog. All mkDefault; opt out with
services.earlyoom.enable = false (README note added).

Verified: V0 (flake check) + V2 — new checks.oom-protection
runNixOSTest, executed: a chunked allocator (686 MB peak, 1 GB VM) is
SIGTERM'd by earlyoom in 0.1 s, a bystander unit survives (the
process-level granularity the module exists for), and systemd-oomd is
asserted inactive. 25 s runtime.

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

README.md

@@ -261,11 +261,16 @@ two tables below are split along exactly that line.
 | `nomarchy.services.restic.enable` | `false` | Opt-in: scheduled daily restic backup (set `.repository` + `.passwordFile`; 7/4/6 retention; list/restore via the `restic-nomarchy` wrapper) |
 
 Beyond the `nomarchy.*` surface, the system layer turns on the usual
-desktop services with `lib.mkDefault` (override natively). One worth
+desktop services with `lib.mkDefault` (override natively). Two worth
 calling out: **`services.fwupd.enable`** is on by default for firmware
 updates via LVFS — it only refreshes metadata, never flashes on its own,
-so run `fwupdmgr update` to apply. Disable with `services.fwupd.enable =
-false;` on machines without real firmware (VMs/headless).
+so run `fwupdmgr update` to apply; disable with `services.fwupd.enable =
+false;` on machines without real firmware (VMs/headless). And
+**`services.earlyoom`** is on by default so running out of memory kills
+the offending process (with a desktop notification) instead of freezing
+the desktop — process-level on purpose, since a Hyprland session is one
+cgroup and systemd-oomd would kill all of it (oomd is disabled
+accordingly). Opt out with `services.earlyoom.enable = false;`.
 
 ## 4. How theming works