Switching niri/umbriel Mod to Alt silently creates keybind collisions

Setting niri input.mod-key = "Alt" (or umbriel general.mod_key = "Alt") turns any existing Mod+Alt+X bind into plain Alt+X, which duplicates Mod+X; Mod+Ctrl+Alt+L collides with a Mod+Ctrl+L lock bind. The Nix build still passes, so grep for Mod+...Alt+ binds and gate them with lib.optionalAttrs (modKey == "Super"). Also check terminal multiplexers (zellij/tmux) using Alt shortcuts: the compositor grabs them first. For per-host choice in a dendritic flake, add a typed fact option (enum Super/Alt) set in the host module and read it in the feature modules rather than overriding home-manager settings from the host file.