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/main.rs

@@ -11,6 +11,7 @@ mod map;
 mod menu;
 mod platform;
 mod props;
+mod rival;
 mod render3d;
 mod save;
 mod settings;
@@ -133,7 +134,7 @@ fn finish_rect(app: &mut App, tool: Tool, start: (i32, i32), mx: f32, my: f32) {
 }
 
 fn new_game(seed: u32, s: &Settings) -> (Game, Cam) {
-    let g = Game::new(seed, s.map_w, s.map_h);
+    let g = Game::new_free(seed, s.map_w, s.map_h, s.difficulty);
     let (cx, cy) = g.map.core;
     let cam = Cam::new(cx as f32 + 1.5, cy as f32 + 1.5);
     (g, cam)