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>

flake.nix

@@ -2,7 +2,7 @@
   description = "Nomarchy - A NixOS-based distribution with Omarchy flavour";
 
   inputs = {
-    nixpkgs.url = "github:nixos/nixpkgs/nixos-25.11";
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-26.05";
     nixos-hardware.url = "github:NixOS/nixos-hardware";
     
     disko = {
@@ -16,7 +16,7 @@
     };
     
     home-manager = {
-      url = "github:nix-community/home-manager/release-25.11";
+      url = "github:nix-community/home-manager/release-26.05";
       inputs.nixpkgs.follows = "nixpkgs";
     };
     
@@ -25,7 +25,9 @@
     # input" warning. Just URL it.
     nix-colors.url = "github:misterio77/nix-colors";
     stylix = {
-      url = "github:danth/stylix";
+      # Pinned to the matching release branch (tested against nixpkgs 26.05)
+      # rather than master, which targets unstable.
+      url = "github:danth/stylix/release-26.05";
       inputs.nixpkgs.follows = "nixpkgs";
     };
     walker = {
@@ -237,6 +239,13 @@
             system.stateVersion = "25.11";
             networking.hostName = "nomarchy";
 
+            # This config is only ever built as a VM (`.build.vm`), where the
+            # qemu-vm module supplies the real root fs (via mkVMOverride). But
+            # nixpkgs 26.05's systemd stage-1 forces `fileSystems."/".fsType`
+            # during plain toplevel evaluation (`nix flake check`), so provide a
+            # placeholder root fs for eval; the VM build overrides it.
+            fileSystems."/" = lib.mkDefault { device = "/dev/disk/by-label/nixos"; fsType = "ext4"; };
+
             virtualisation.vmVariant.virtualisation.memorySize = 4096;
             virtualisation.vmVariant.virtualisation.cores = 2;