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>

agent/JOURNAL.md

@@ -17,6 +17,22 @@ Template:
 
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+## 2026-07-10 — #76 zram slice: compressed-RAM swap default
+- **Task:** BACKLOG #76 (`[big]`) — split; took the low-risk zram-default slice.
+- **Did:** `zramSwap` on by default in `modules/nixos/oom.nix` (zstd, 50% RAM,
+  priority 100) as the memory-pressure layer before earlyoom, following the
+  earlyoom mkDefault precedent (no toggle). High priority keeps day-to-day
+  paging in zram so a disk swapfile stays reserved for #76's hibernate image.
+  Added `checks.zram-swap` runNixOSTest (imports oom.nix). Split #76 in
+  BACKLOG → remainder is the hibernation half (swapfile/resume/installer).
+- **Verified:** **V2** — `nix build .#checks…zram-swap` PASS: /dev/zram0 active
+  in /proc/swaps, `[zstd]` comp_algorithm, priority 100. **V1** full distro
+  toplevel builds. **V0** flake check --no-build green.
+- **Pending:** hibernation half of #76 still open (needs hibernate→resume VM
+  test + V3 laptop). No V3 for zram (under-load tuning is secondary).
+- **Next suggestion:** #76 hibernation half — design default swapSize + the
+  resume/installer wiring; it's the boot-critical, V2-hibernate + V3 part.
+
 ## 2026-07-10 — #41 hardware-confirmed: OpenFile portal floats (V3 close)
 - **Task:** BACKLOG #41 — final on-hardware confirmation (Bernardo reported PASS).
 - **Did:** #41 already out of NEXT + portal rule shipped