feat(memory): enable zram compressed-RAM swap by default (#76 slice)

First slice of BACKLOG #76 (hibernation + zram, split): zramSwap on by
default in modules/nixos/oom.nix — the memory-pressure layer before
earlyoom. zstd @ 50% RAM (nixpkgs defaults, explicit for reproducibility),
priority 100 so day-to-day paging fills zram first and any disk swapfile
stays reserved for #76's hibernate image (hibernation can't resume from
volatile zram). Follows the earlyoom mkDefault precedent — no toggle.

Adds checks.zram-swap runNixOSTest importing oom.nix, asserting the device
is an active swap, uses [zstd], and carries priority 100.

Verified: V2 checks.zram-swap PASS (/dev/zram0 in /proc/swaps, zstd,
prio 100); V1 full distro toplevel builds; V0 flake check --no-build green.
#76 remainder (disk swapfile default + resume + installer) still open.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/nixos/oom.nix

@@ -14,9 +14,28 @@
 # the build step, keep the session". nixpkgs default-enables oomd in an
 # inert state (no slices monitored); it's disabled outright below so
 # there is exactly one owner of the OOM story.
+#
+# zram sits one layer earlier: a compressed RAM swap that absorbs
+# day-to-day memory pressure (a browser's idle tabs, a big eval's cold
+# pages) before it ever reaches the earlyoom threshold — more headroom
+# on the same RAM, no disk. High swap priority so the kernel fills zram
+# first; any real disk swapfile stays reserved for the hibernate image
+# (BACKLOG #76's other half — hibernation can't resume from volatile
+# zram, so day-to-day paging must not consume the disk swap). zstd @ 50%
+# RAM are the nixpkgs defaults, kept explicit for reproducibility.
 { lib, ... }:
 
 {
+  zramSwap = {
+    enable = lib.mkDefault true;
+    algorithm = lib.mkDefault "zstd";
+    memoryPercent = lib.mkDefault 50;
+    # Beats any disk swapfile's auto-assigned (negative) priority, so
+    # day-to-day paging lands in zram and the disk swap is left for
+    # hibernation. See the header note and BACKLOG #76.
+    priority = lib.mkDefault 100;
+  };
+
   services.earlyoom = {
     enable = lib.mkDefault true;