refactor(system): move programs.uwsm to its own session.nix

The session-manager wiring (uwsm + the Hyprland Wayland-compositor
entry that gives Hyprland a proper graphical-session.target so user
services like nomarchy-wallpaper, walker, and elephant chain off it)
had lived in core/system/virtualization.nix by historical accident —
loaded unconditionally on every install, nothing to do with libvirt
or docker.

Lifted into a dedicated core/system/session.nix and imported from
core/system/default.nix between systemd.nix and virtualization.nix.
virtualization.nix now contains only the libvirt + docker branches
its filename implies.

`nix flake check --no-build` clean. No behaviour change.

core/system/default.nix

@@ -9,6 +9,7 @@
     ./nix.nix
     ./scripts.nix
     ./systemd.nix
+    ./session.nix
     ./virtualization.nix
     ./fonts.nix
     ./hardware.nix