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/campaign.rs

@@ -505,6 +505,7 @@ pub fn start_mission(i: usize, carry: Option<&Carry>) -> Game {
     }
     g.place_dwarf_caves(d.seed);
     g.init_research(Some(i));
+    g.init_fog();
     let heart = d.gate || d.forts.iter().any(|f| f.heart);
     if !heart {
         g.gate_hp = 0.0;