Fog of war: unexplored darkness until the dungeon's ground, creatures or towers come close

- explored tiles stay known (saved compactly as bits), enemies are only drawn while in sight; sight runs along open ground, walls block it
- unexplored tiles are black blocks; units, towers, torches, furniture, portals, gate and rival cores in the dark are hidden; minimap and tile info too
- the nearest portal is known from the start; the seer's eye opens a wide area for 20 seconds without remembering it

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

src/rival.rs

@@ -86,6 +86,7 @@ impl Game {
         g.msg("msg.welcome2", vec![]);
         g.msg(if rivals == 0 { "msg.welcome_humans" } else if humans { "msg.welcome_both" } else { "msg.welcome_rivals" }, vec![rivals.to_string()]);
         g.init_research(None);
+        g.init_fog();
         g.place_dwarf_caves(seed);
         g
     }