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/default.nix

@@ -21,6 +21,7 @@
     ./snapper.nix
     ./laptop.nix
     ./desktop.nix
+    ./accessibility.nix
     ./hibernate.nix
     ./containers.nix
     ./pam.nix