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/settings.rs

@@ -147,6 +147,7 @@ pub struct Settings {
     // game
     pub game_speed: f32, // 1 or 2
     pub hints: bool,
+    pub difficulty: u8, // rival masters in free games: 0 easy, 1 medium, 2 hard
     pub map_w: usize, // size of new maps (64..256)
     pub map_h: usize,
     // key bindings: action id -> key name
@@ -172,6 +173,7 @@ impl Default for Settings {
             touch_ui: 0,
             game_speed: 1.0,
             hints: true,
+            difficulty: 1,
             map_w: 64,
             map_h: 64,
             keys: BTreeMap::new(),