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>

lang/en.lang

@@ -827,3 +827,4 @@ tool.plague.desc = A sickness that eats at the enemies around the spot and jumps
 tool.quake.label = Quake
 tool.quake.name = Quake
 tool.quake.desc = The ground shakes: enemies around the spot are hurt and stunned for two seconds. Your creatures keep their footing.
+tile.unexplored = Unexplored darkness