Umbriel window gap lives in layout.gap, borders add to it

In the umbriel Wayland compositor's home-manager settings, the space between tiled windows is programs.umbriel.settings.layout.gap (pixels). The visible band between two windows is gap plus both windows' appearance.border_width, so a user asking for a thinner separator may also want border_width lowered. If niri runs alongside, its equivalent is layout.gaps, which must be changed separately or the two compositors drift.