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/game.rs

@@ -483,6 +483,8 @@ pub struct Game {
     #[serde(default)]
     pub reveals: Vec<(f32, f32, f32)>, // seer's eye: x, y, seconds left
     #[serde(skip)]
+    pub fog_t: f32, // fog of war: seconds until the next update
+    #[serde(skip)]
     pub forge_work: f32, // work done in the forge this frame
     #[serde(skip)]
     pub demo: Option<crate::demo::Demo>, // the endless bridge battle behind the title menu
@@ -576,6 +578,7 @@ impl Game {
         g.guard_treasures();
         g.place_dwarf_caves(seed);
         g.init_research(None);
+        g.init_fog();
         g.msg("msg.welcome1", vec![]);
         g.msg("msg.welcome2", vec![]);
         g.msg("msg.welcome3", vec![]);
@@ -652,6 +655,7 @@ impl Game {
             rollers: Vec::new(),
             clouds: Vec::new(),
             forge_work: 0.0,
+            fog_t: 0.0,
             reveals: Vec::new(),
             known: Vec::new(),
             researchable: Vec::new(),
@@ -1105,6 +1109,7 @@ impl Game {
         self.update_forge(dt);
         self.update_research();
         self.update_reveals(dt);
+        self.update_fog(dt);
         if !self.rivals.is_empty() {
             self.update_rivals(dt);
         }