feat: bump to NixOS 26.05 (Yarara) — WIP, desktop unverified on hardware

Bumps nixpkgs nixos-25.11 -> nixos-26.05, home-manager release-25.11 ->
release-26.05, and pins stylix to release-26.05 (was unpinned master).
`nix flake check --no-build` and `nomarchy-eval-matrix` are both clean and
the full system builds (nixos-system-nomarchy-26.05).

Breaking-change fixes required by 26.05:
- stylix-compat: drop the `programs.neovim.initLua` shim — HM 26.05 declares
  it natively, so the shim was a duplicate-option error.
- swww was renamed to awww (binaries gone, no `init` subcommand). Dropped swww
  entirely and unified the wallpaper path on swaybg (the daemon this distro
  actually runs): nomarchy-theme-bg-next now delegates to nomarchy-theme-bg-set,
  swaybg added to the theme-engine script deps, swww removed from package lists.
- default config: add a placeholder `fileSystems."/"` — 26.05's systemd stage-1
  forces fsType during plain toplevel eval (the VM build overrides it).
- services.resolved: extraConfig removed, domains/fallbackDns renamed -> migrated
  to settings.Resolve.* (RFC42).
- systemd.sleep.extraConfig removed -> settings.Sleep.HibernateDelaySec (RFC42).
- X11 'virtio' video driver removed -> dropped from vm-guest.nix and
  nomarchy-live.nix (modesetting drives virtio-gpu).
- nomarchy-eval-matrix: define /persist + /home in the impermanence scenarios
  (26.05 maps fsType over every neededForBoot filesystem).
- README/MIGRATION/installer: stale swww references -> swaybg.

KNOWN BLOCKER (why this is on a branch, not main):
In a headless QEMU VM, Hyprland boots and inits GL (virgl) fine but is
OOM-killed after ~19s (3.4 GB peak). Likely a no-display-consumer artifact of
headless rendering (unthrottled frames), but a real Hyprland-26.05 memory
regression can't be ruled out without a real display. Verify on real hardware
(a fresh ISO install) before merging to main; if the desktop is stable there,
the OOM was the headless VM and this can merge.

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

themes/engine/scripts/nomarchy-theme-bg-next

@@ -1,8 +1,8 @@
 #!/usr/bin/env bash
 set -e
 
-# Cycles through the background images available for the current theme.
-# Declarative + Hybrid (instant swww) for Nomarchy NixOS.
+# Cycles through the background images available for the current theme,
+# then applies the pick via nomarchy-theme-bg-set (see the note at the end).
 
 STATE_DIR="$HOME/.config/nomarchy"
 STATE_FILE="$STATE_DIR/state.json"
@@ -45,13 +45,11 @@ done
 NEXT_INDEX=$(((INDEX + 1) % TOTAL))
 NEW_BG="$BG_DIR/${BACKGROUNDS[$NEXT_INDEX]}"
 
-nomarchy-state-write wallpaper "$NEW_BG"
-
-# Instant feedback via swww
-if pgrep -x swww-daemon >/dev/null; then
-    swww img "$NEW_BG" --transition-type outer --transition-pos 0.85,0.97 --transition-step 90
-else
-    swww init && swww img "$NEW_BG"
-fi
-
-echo "Background set to $NEW_BG declaratively."
+# Apply through the shared setter: it persists state.json, updates the
+# ~/.config/nomarchy/current/background symlink, and restarts swaybg — the
+# single wallpaper daemon this distro actually runs. Previously this script
+# drove swww directly, but it gated on `swww-daemon` (never running, since
+# swaybg owns the wallpaper), so it spawned a second daemon competing with
+# swaybg. swww was also renamed to awww and dropped its `init` subcommand in
+# nixpkgs 26.05, so unifying on swaybg fixes both at once.
+exec nomarchy-theme-bg-set "$NEW_BG"