test(hibernate): #76 checks.hibernate-swapfile — verify swapfile+offset in a VM

Push #76's swapfile-resume residual into a VM as far as headlessly possible.
A full hibernate→resume from a RUNTIME swapfile turned out not to be
headless-reachable: systemd reports CanHibernate=no for a swap that isn't
declared in the boot config, and a runtime-computed resume_offset can't be
fed into the boot cmdline (the genuine chicken-and-egg). So the check now
verifies, bounded and deterministically, what the partition test
(checks.hibernate) can't and what a VM CAN prove: the installer's btrfs
@swap swapfile is built correctly and its offset is valid — NOCOW
mkswapfile, file-type swapon, a good `map-swapfile -r` offset the kernel
accepts (/sys/power/resume_offset), and zram (prio 100) above it.

The literal encrypted-swapfile power-cycle + LUKS-initrd unlock stays a V3
laptop check (the dev box runs exactly this layout, /proc/cmdline verified).

(First cut had an unbounded wait_for_shutdown that hung on the CanHibernate
refusal; rewritten with a thread-bounded wait + a CanHibernate pre-check,
which gave the clean verdict. Device is /dev/vdb without useNixStoreImage.)

Verification: V2 PASS (nix build .#checks.x86_64-linux.hibernate-swapfile,
fresh, exit 0); flake check --no-build green. #76 residual narrowed in
BACKLOG/ROADMAP.

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

flake.nix

@@ -752,6 +752,66 @@
             '';
           };
 
+          # The installer's BTRFS @swap SWAPFILE setup (#76) — the parts the
+          # partition hibernate test (checks.hibernate) can't reach. A full
+          # hibernate→resume from a *runtime* swapfile isn't reachable
+          # headlessly: systemd reports CanHibernate=no unless the swap is
+          # declared in the boot config, and feeding a runtime-computed
+          # resume_offset into the boot cmdline is a genuine chicken-and-egg.
+          # So the power-cycle stays a real-hardware check (the dev box runs
+          # exactly this layout — /proc/cmdline verified). What IS verified
+          # here, bounded and deterministic, is that the swapfile is built
+          # correctly and its resume offset is valid: NOCOW `mkswapfile`, a
+          # good `map-swapfile -r` offset the kernel accepts, swapon as a file,
+          # and zram (prio 100) above it so the image lands on disk, not zram.
+          hibernate-swapfile = pkgs.testers.runNixOSTest {
+            name = "nomarchy-hibernate-swapfile";
+            nodes.machine = { config, lib, pkgs, ... }: {
+              imports = [ ./modules/nixos/oom.nix ];  # zram default-on (#76)
+              virtualisation.memorySize = 2048;
+              # A dedicated disk for the btrfs @swap subvolume.
+              virtualisation.emptyDiskImages = [ (4 * config.virtualisation.memorySize) ];
+              environment.systemPackages = [ pkgs.btrfs-progs ];
+            };
+            testScript = ''
+              machine.wait_for_unit("default.target")
+              machine.wait_until_succeeds("grep -q /dev/zram0 /proc/swaps", timeout=60)
+
+              # Reproduce the installer layout: btrfs, @swap subvolume, then a
+              # NOCOW swapfile via `btrfs filesystem mkswapfile`.
+              # The lone empty data disk (root is vda; no separate store disk
+              # here since useNixStoreImage is off).
+              machine.succeed("mkfs.btrfs -f /dev/vdb")
+              machine.succeed(
+                  "mkdir -p /mnt-btrfs && mount /dev/vdb /mnt-btrfs "
+                  "&& btrfs subvolume create /mnt-btrfs/@swap && umount /mnt-btrfs"
+              )
+              machine.succeed("mkdir -p /swap && mount -o subvol=@swap /dev/vdb /swap")
+              machine.succeed("btrfs filesystem mkswapfile --size 3g --uuid clear /swap/swapfile")
+              # NOCOW is mandatory — a copy-on-write swapfile corrupts on btrfs.
+              machine.succeed("lsattr /swap/swapfile | grep -q C")
+              machine.succeed("swapon /swap/swapfile")
+              machine.succeed(
+                  "awk '$1==\"/swap/swapfile\" && $2==\"file\"{f=1} END{exit !f}' /proc/swaps"
+              )
+
+              # The Nomarchy-specific offset math (patch-template bakes this
+              # into resume_offset): a valid physical offset the kernel accepts.
+              offset = machine.succeed(
+                  "btrfs inspect-internal map-swapfile -r /swap/swapfile"
+              ).strip()
+              assert offset.isdigit() and int(offset) > 0, f"bad resume offset: {offset!r}"
+              machine.succeed(f"echo {offset} > /sys/power/resume_offset")
+
+              # zram (prio 100) must sit above the disk swapfile, so a hibernate
+              # image would land on disk, not volatile zram.
+              machine.succeed(
+                  "awk '$1==\"/dev/zram0\"&&$5==100{z=1} "
+                  "$1==\"/swap/swapfile\"&&$5<100{d=1} END{exit !(z&&d)}' /proc/swaps"
+              )
+            '';
+          };
+
           # Auto time-of-day theme (#79, VISION § D): simulate crossing
           # sunset and sunrise by moving the VM clock, and assert
           # `nomarchy-theme-sync auto` switches the active theme accordingly