Function mode_toggle_ex_command_spec 

pub fn mode_toggle_ex_command_spec(mode_name: &str) -> ExCommandSpec

Build the auto-generated :<mode-name> mode-toggle ex-command spec. The apply returns crate::effect::Effect::ToggleMode; the host routes that to toggle_mode_by_name, flipping the mode on the active buffer. Shared by boot (native modes, register_mode_toggle_commands) AND the plugin modes-seam drain, so a plugin-registered mode gets an IDENTICAL :<mode> toggle surface. The caller registers it under the right provenance: Builtin for native modes; SourceLayer::Plugin(id) for plugin modes, so unload reverses it. Takes no arguments.