Configurable map size (64-256, non-square), one portal per 64x64 tiles, guarded treasure caves, glowing dig-mark blocks

- New Game dialog with map width/height (steps of 32), remembered in settings
- map dimensions are per game; renderer chunks, minimap and camera follow them
- content scales with area; grace period grows with the distance to the Hero Gate
- treasure caves with rich gold, guarded by hero squads that hold their post
- marked blocks: translucent yellow box with outline on all edges
- savegame v2: packed run-length encoded tiles (256x256 map ~320 KB), v1 still loads
- tests: savegame round trip on 96x128, 256x256 generation/simulation/save

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

src/settings.rs

@@ -145,6 +145,8 @@ pub struct Settings {
     // game
     pub game_speed: f32, // 1 or 2
     pub hints: bool,
+    pub map_w: usize, // size of new maps (64..256)
+    pub map_h: usize,
     // key bindings: action id -> key name
     pub keys: BTreeMap<String, String>,
 }
@@ -166,6 +168,8 @@ impl Default for Settings {
             ui_scale: 2,
             game_speed: 1.0,
             hints: true,
+            map_w: 64,
+            map_h: 64,
             keys: BTreeMap::new(),
         }
     }