Free game against rival dungeon masters (green, blue) and/or the humans

- One opponent on maps up to 128x128, two on bigger maps; with a chance of one in two one of
  them is the humans' gate instead of a rival master
- Simplified rival masters (rival.rs): own soul core and dungeon (lair, treasury to loot) that
  grows ring by ring, creatures appear and get stronger over time, guard the dungeon and attack
  in groups - the player, the other master or the humans' gate; a fallen core puts the master
  out and frees its ground
- The humans attack everybody: waves now and then march on a rival master
- Sides for units: all other sides are enemies; towers and contested ground follow them; the
  hand cannot pick up rival creatures; the flag next to a rival core attacks it
- Difficulty (easy / medium / hard) in the New Game dialog
- Colours: green and blue floors, cores, unit rings, minimap, HUD bars of the rival cores
- Victory in free games: every rival core and the gate (if any) destroyed
- Tests for setups, growth and attacks, elimination and victory, humans against rivals

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/menu.rs

@@ -189,6 +189,7 @@ enum Btn {
     Change(usize, i32),
     MapW(i32),
     MapH(i32),
+    Difficulty(i32),
     Start,
     Key(usize),
     ResetKeys,
@@ -405,6 +406,10 @@ impl Menu {
                 settings.map_w = (settings.map_w as i32 + d * 32).clamp(MIN_SIZE as i32, MAX_SIZE as i32) as usize;
                 MenuAction::SettingsChanged
             }
+            Some(Btn::Difficulty(d)) => {
+                settings.difficulty = (settings.difficulty as i32 + d).rem_euclid(3) as u8;
+                MenuAction::SettingsChanged
+            }
             Some(Btn::MapH(d)) => {
                 settings.map_h = (settings.map_h as i32 + d * 32).clamp(MIN_SIZE as i32, MAX_SIZE as i32) as usize;
                 MenuAction::SettingsChanged
@@ -631,13 +636,15 @@ impl Menu {
         let sw = sw();
         let sh = sh();
         let (mx, my) = mouse();
-        let cols = 44;
-        let rows = 17;
-        let x = grid_center(sw, cols as f32 * CW());
+        let cols = 48.min((sw / CW()) as i32);
+        let rows = 22;
+        let x = grid_center(sw, cols as f32 * CW()).max(0.0);
         let y = (grid_center_y(sh, rows as f32 * CH())).max(0.0);
         gfx.window_titled(x, y, cols, rows, S(), true, FRAME_HI, Color::new(0.05, 0.03, 0.04, 0.97), &tr("menu.new"), FRAME_HI);
-        let lines = [("newgame.width", s.map_w, 0), ("newgame.height", s.map_h, 1)];
-        for (k, &(key, val, which)) in lines.iter().enumerate() {
+        let diff = tr(&format!("diff.{}", s.difficulty.min(2)));
+        let lines = [("newgame.width", s.map_w.to_string(), 0), ("newgame.height", s.map_h.to_string(), 1), ("newgame.difficulty", diff, 2)];
+        for (k, (key, val, which)) in lines.iter().enumerate() {
+            let (key, which) = (*key, *which);
             let by = y + CH() * (2.0 + k as f32 * 2.5);
             gfx.text(&tr(key), x + CW() * 3.0, by + CH() * 0.5, S(), TXT);
             let vw = 14.0 * CW();
@@ -646,9 +653,13 @@ impl Menu {
             let right = Rect::new(vx + vw - CW() * 2.5, by, CW() * 2.5, CH() * 2.0);
             button(gfx, left, "<", left.contains(vec2(mx, my)), FRAME);
             button(gfx, right, ">", right.contains(vec2(mx, my)), FRAME);
-            let v = val.to_string();
+            let v = val.clone();
             gfx.text(&v, vx + (vw - gfx.measure(&v, S())) * 0.5, by + CH() * 0.5, S(), GOLD);
-            let (l, r) = if which == 0 { (Btn::MapW(-1), Btn::MapW(1)) } else { (Btn::MapH(-1), Btn::MapH(1)) };
+            let (l, r) = match which {
+                0 => (Btn::MapW(-1), Btn::MapW(1)),
+                1 => (Btn::MapH(-1), Btn::MapH(1)),
+                _ => (Btn::Difficulty(-1), Btn::Difficulty(1)),
+            };
             self.buttons.push((left, l));
             self.buttons.push((right, r));
         }
@@ -656,9 +667,14 @@ impl Menu {
         let area = (s.map_w * s.map_h) as f32 / 4096.0;
         let portals = (area.round() as usize).max(1);
         let info = trf("newgame.info", &[portals.to_string(), (portals - 1).to_string()]);
-        gfx.text(&info, x + CW() * 3.0, y + CH() * 7.5, S(), DIM);
+        gfx.text(&info, x + CW() * 3.0, y + CH() * 10.0, S(), DIM);
+        let opp = tr(if s.map_w * s.map_h > 128 * 128 { "newgame.opponents2" } else { "newgame.opponents1" });
+        let maxc = (cols - 6) as usize;
+        for (k, l) in wrap(&opp, maxc).iter().take(2).enumerate() {
+            gfx.text(l, x + CW() * 3.0, y + CH() * (11.0 + k as f32), S(), DIM);
+        }
         let w = (cols - 6) as f32 * CW();
-        self.column(gfx, x + CW() * 3.0, y + CH() * 9.5, w, &[(Btn::Start, tr("newgame.start")), (Btn::Back, tr("menu.back"))]);
+        self.column(gfx, x + CW() * 3.0, y + CH() * 14.0, w, &[(Btn::Start, tr("newgame.start")), (Btn::Back, tr("menu.back"))]);
     }
 
     fn draw_pause(&mut self, gfx: &Gfx) {