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>

flake.nix

@@ -659,6 +659,31 @@
             '';
           };
 
+          # zram compressed-RAM swap (oom.nix, BACKLOG #76 slice): enabled
+          # by default as the memory-pressure layer *before* earlyoom. Prove
+          # the device comes up as an active swap, uses zstd, and carries the
+          # high priority that keeps day-to-day paging off any disk swapfile
+          # (which #76 reserves for the hibernate image). Imports oom.nix
+          # directly so the check tracks the module, not a hand-copy.
+          zram-swap = pkgs.testers.runNixOSTest {
+            name = "nomarchy-zram-swap";
+            nodes.machine = { ... }: {
+              imports = [ ./modules/nixos/oom.nix ];
+              virtualisation.memorySize = 1024;
+            };
+            testScript = ''
+              # zram-generator brings the device up as swap shortly after boot.
+              machine.wait_until_succeeds("grep -q /dev/zram0 /proc/swaps", timeout=60)
+              # Configured compression algorithm is the selected one ([zstd]).
+              machine.succeed("grep -q '\\[zstd\\]' /sys/block/zram0/comp_algorithm")
+              # High priority (100) so the kernel fills zram before any disk
+              # swap — the ordering #76's hibernate image depends on.
+              machine.succeed(
+                  "awk '$1==\"/dev/zram0\" && $5==100 {ok=1} END{exit !ok}' /proc/swaps"
+              )
+            '';
+          };
+
           # Snapshot GUI canary: btrfs-assistant crashes in libbtrfsutil's
           # UNPRIVILEGED subvolume iteration (btrfs-progs 6.17.1, fixed
           # upstream after 6.17.1) but runs fine as root — which is how the