feat(system): add accessibility preset module

Opt-in `nomarchy.system.accessibility.enable` (default false —
accessibility is a personal preference, not hardware-derived). Wires
`services.gnome.at-spi2-core`, installs `pkgs.orca`, and sets
`XCURSOR_SIZE` to a configurable `accessibility.cursorSize` (default
32, up from NixOS's 24).

The original roadmap entry bundled Hyprland-side bits (slower
key-repeat, Orca launch keybinding, high-contrast palette). Those
require touching home-manager / theme files and a new palette
directory; split into a separate Next-column row so the system-side
preset ships now and the desktop integration follows independently.

core/system/accessibility.nix

@@ -0,0 +1,14 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.nomarchy.system.accessibility;
+in
+{
+  config = lib.mkIf cfg.enable {
+    services.gnome.at-spi2-core.enable = lib.mkDefault true;
+
+    environment.systemPackages = [ pkgs.orca ];
+
+    environment.variables.XCURSOR_SIZE = toString cfg.cursorSize;
+  };
+}