feat(mirada): nmaster, promover a maestra y smart gaps (estilo dwm)

Tanda de funciones de tiling WM, toda pura (mirada-layout/brain), sin
tocar el protocolo:

- nmaster: LayoutParams.master_count — cuántas ventanas van en el área
  maestra. MasterStack y CenteredMaster apilan N maestras; sin pila, las
  maestras llenan la pantalla. Acciones inc-master/dec-master (Super+,
  Super+.), acotadas 1..9.
- Promover a maestra: Workspace::promote_focused lleva la ventana
  enfocada al puesto 0. Acción promote-to-master (Super+Return).
- Smart gaps: una sola ventana se tesela a sangre, sin margen.

combo_string del compositor canoniza ahora teclas con nombre (Return,
Tab, F5, flechas…) vía xkb::keysym_get_name, no sólo caracteres
imprimibles — sin eso Super+Return no sería un atajo expresable.

Cableado en keymap por defecto, HUD de mirada y mirada-ctl. Verificado
end-to-end con headless-ctl. mirada-layout 26->30, mirada-brain 39->41.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/modules/mirada/mirada-brain/src/desktop.rs

@@ -190,6 +190,12 @@ impl Desktop {
             }
             DesktopAction::GrowMaster => self.nudge_master(0.05),
             DesktopAction::ShrinkMaster => self.nudge_master(-0.05),
+            DesktopAction::IncMaster => self.nudge_master_count(1),
+            DesktopAction::DecMaster => self.nudge_master_count(-1),
+            DesktopAction::PromoteToMaster => {
+                self.workspaces[self.active].promote_focused();
+                self.relayout()
+            }
             DesktopAction::SwitchWorkspace(n) => {
                 if n < self.workspaces.len() && n != self.active {
                     self.active = n;
@@ -224,6 +230,14 @@ impl Desktop {
         self.relayout()
     }
 
+    /// Ajusta `nmaster` del escritorio activo, acotado a `1..=9`.
+    fn nudge_master_count(&mut self, delta: i32) -> Vec<BrainCommand> {
+        let ws = &mut self.workspaces[self.active];
+        let n = (ws.params().master_count as i32 + delta).clamp(1, 9) as usize;
+        ws.set_master_count(n);
+        self.relayout()
+    }
+
     /// Recalcula la geometría del escritorio activo y la empaqueta en un
     /// [`BrainCommand::Place`]. Sin salida conectada, no hay nada que
     /// colocar.
@@ -468,6 +482,32 @@ mod tests {
         assert!((d.active_workspace().params().master_ratio - r0).abs() < 1e-6);
     }
 
+    #[test]
+    fn inc_and_dec_master_adjust_nmaster() {
+        let mut d = desktop_with_screen();
+        for id in [1, 2, 3] {
+            open(&mut d, id);
+        }
+        assert_eq!(d.active_workspace().params().master_count, 1);
+        d.apply(DesktopAction::IncMaster);
+        assert_eq!(d.active_workspace().params().master_count, 2);
+        d.apply(DesktopAction::DecMaster);
+        d.apply(DesktopAction::DecMaster); // no baja de 1
+        assert_eq!(d.active_workspace().params().master_count, 1);
+    }
+
+    #[test]
+    fn promote_to_master_brings_the_focused_window_to_the_front() {
+        let mut d = desktop_with_screen();
+        for id in [1, 2, 3] {
+            open(&mut d, id);
+        }
+        d.apply(DesktopAction::FocusWindow(3));
+        d.apply(DesktopAction::PromoteToMaster);
+        assert_eq!(d.active_workspace().windows()[0], 3);
+        assert_eq!(d.focused_window(), Some(3));
+    }
+
     #[test]
     fn master_ratio_stays_within_bounds() {
         let mut d = desktop_with_screen();