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/options.nix

@@ -126,6 +126,25 @@
       };
     };
 
+    accessibility = {
+      enable = lib.mkEnableOption ''
+        Accessibility preset: AT-SPI2 framework, the Orca screen reader
+        on PATH, and a larger default cursor size. Off by default —
+        accessibility is a personal preference, not a hardware-derived
+        signal. The Hyprland-side keybinding to launch Orca is a
+        separate roadmap item.
+      '';
+      cursorSize = lib.mkOption {
+        type = lib.types.int;
+        default = 32;
+        description = ''
+          XCURSOR_SIZE when accessibility is on. NixOS default is 24;
+          32 is a safer floor for low-vision users. Bump to 48 if the
+          user explicitly asks.
+        '';
+      };
+    };
+
     containers = {
       enable = lib.mkEnableOption ''
         Rootless Podman with Docker compatibility (`docker` → `podman`),